Where the best scenery is underwater. Celebration of nation's bicentennial could include snorkeling at the Great Barrier Reef, topped off by numerous other activities

I AM not easily impressed, mind you. But I had an experience not long ago that ranks right up there with my wedding and the birth of my children. I snorkeled on the Great Barrier Reef. If you're planning a trip to World Expo 88 in Brisbane this year - or even if you're not - I would suggest you head up Queensland way and experience your own total immersion in the waters around the reef. It's the ultimate side trip from anywhere in Australia.

Stretching for more than 2,000 miles along the northeastern coast, the ``Eighth Wonder of the World'' isn't a single reef. It is, instead, a series of more than 2,600 coral shoals, cays, and outcrops plus about 300 islands.

If you have only two or three days, you might stay on one of those islands - maybe the popular Dunk Island or exclusive Bedarra. But if you have more time, you could headquarter on the mainland and take commercial tours out to the reef.

Quicksilver Tours, here in Cairns, offers one of the best excursions to the outer barrier reef. You're picked up at your hotel and taken by motorcoach to Port Douglas, about 35 miles north of Cairns, passing a string of tawny sand beaches and sugar cane fields en route.

At Port Douglas you board the boat for a 1-hour ride to Agincourt Reef. Once there, you can take submarine rides, look at the fish from an enclosed underwater viewing room, or snorkel.

When you reach the outer reef, however, it's only by snorkeling - or scuba diving for the more adventurous - that you maximize the reef experience. Blue angelfish with their yellow-edged gills, pink staghorn coral, and giant clams five feet across seem close enough to touch, despite the fact that they're 30 feet below.

About 1,600 species of fish; 4,000 kinds of mollusks such as clams and snails; myriad sponges, worms, crustaceans, starfish, and sea urchins thrive in this fascinating waterworld. The reef itself is composed of about 400 types of hard and soft corals, so that every second another kaleidoscope of color appears in front of your eyes.

Cairns a convenient base for trips

Cairns, about 1,500 kilometers (about 930 miles) north of Brisbane, is probably the best place to stay on the mainland. Townsville, the other major population center in the Barrier Reef area, is flat and dry. Lushly green Cairns, by contrast, sits on Trinity Inlet, with the Great Dividing Range as a backdrop and the mountains of Admiralty Island rising across the water.

Since the surroundings are so delightful, you'll want a room with a view. The top hotel in the area is the Cairns International Hilton, which opened in November 1987. Breakfast on the balcony of your room overlooking the water, tea in the elegantly contemporary lobby, and lounging around the pool under the palm trees could make a holiday memorable even if there was no reef.

An even larger hostelry, the Park Royal, is in final construction stages, and a Holiday Inn is in the works. A half dozen other hotels face the Esplanade.

In addition to gorgeous scenery, another bonus of making Cairns your base is the wealth of things to see and do here. For example, you can take trips to the islands of the inner reef, by commercial tour, or by sailboat. A great bargain, especially if you have your own snorkel gear, is the round-trip boat to Green Island for $14 (Australian; about US$11, children half fare), either by launch or Big Cat. The launch leaves Hayles Wharf at 9 a.m. and returns at 4:30 p.m., giving you the whole day to explore the island, picnic, snorkel, and lie on the beach.

Though you can take commercial boat tours to Fitzroy Island, locals advise choosing Green Island, where there is more to do. Fitzroy is steeper, they say, and it's harder to get around.

If you'd like to spend more time on the water, consider a two-hour calm water cruise around Cairns Harbour and Admiralty Island on the paddle wheeler SS Louisa, said to be the first steamship built in Australia. Another two-hour ride following the same route aboard the cruise Terri-Too includes tea or snack lunch. Both tours leave from Marlin Jetty.

For still more water fun, rent a catamaran, canoe, or sailboard. Jet skis and aquabikes that you pedal through the water are available, too.

It's easy to get around the Cairns area on a reliable public transportation system. The city bus service, which costs an average of $A1 per trip (US, 80 cents), can take you to such attractions as the Botanical Gardens, Centenary Lakes, Mt. Whitfield Bush Walks, and Earlville Shoppingtown. Sun worshipers will find the Beach Bus handy, as it leaves the city several times a day for the beaches north of town. Remember, too, that most buses don't make evening runs.

Be sure to allow time for a stroll along the water on the Esplanade at high tide, since the area's a mud flat when the tide is out. It's the time when you'll see the most egrets, pelicans, wandering tattlers, sandpipers, and other shore birds. Day trips to unusual spots available

From Cairns, you can take a day trip to Kuranda, a tourist center set in the middle of a rain forest about 42 km (26 miles) northwest of Cairns. The most outstanding attraction in town is the Aboriginal Tjapukai Dance Theatre, in which seven Aboriginal men, their bodies painted with traditional designs, dance and tell of their ancient herit-age.

Nearby, bright orange cruisers, red lacewings, electric blue Ulysses, and dozens of other species of lepidoptera flutter about the Australian Butterfly Sanctuary, the largest in the world.

Across the street at the Noctarium, daytime has been turned into night so that visitors can watch species of creatures that only come out at night - spectacled flying fox, echnida, ringtail possums, sugar gliders, and northern mastiff bats.

Another popular day trip is a boat ride on the Daintree River, complete with butterflies, exotic birds, and crocodiles.

As far as night life goes, you'll probably have to settle for hotel lounge entertainment, which usually features solo musicians or groups playing everything from country to rock.

Occasionally, if your timing is right, you'll get in on an event like last November's Stevie Wonder concert at the Cairns Showgrounds.

But you won't need after-dark entertainment. After all, you'll want to turn in so you'll be ready to rise early to snorkel on the Great Barrier Reef again. Once is definitely not enough.

If you go

Here are a few inexpensive things to do in Cairns:

The Cairns Chamber Orchestra presents free concerts in a downtown shopping center on Friday nights.

Since many of the parks have barbecues and picnic tables, go to the supermarkets for provisions. Kiwi fruit, lamb chops, and cheeses are especially inexpensive.

Hop on the Red Bus at City Place in the heart of downtown and you can spend the day sightseeing for A$8 (about US$6). Get off the bus at any of the eight stops and climb back on when the bus comes around again on its hourly schedule. Be sure to wear a bathing suit under your shorts, as the most delightful of the attractions along the route is the Freshwater Creek swimming hole, set in the midst of canefields and lush tropical mountains. Another winner is the jungle walk on a raised wooden pathway between the Botanic Gardens and Centenary Lakes.

If it's raining, stroll around Earlville Shoppingtown, the area's largest shopping center, for an afternoon of free entertainment - provided you can resist buying anything.

For added fun in Cairns, try ...

The breakfast buffet at posh Cairns International Hilton, which opened Nov. 2, 1987. At A$16 (about US$12), this lavish spread of cold meats, breakfast dishes, cheeses, fruits, and pastries tastes delicious. The view through the palm trees of boats moored in the water adds a delightful ambiance.

Helicopter flights over the Cairns area, rain forests, Stoney Creek Falls, and the beaches north of town. Priced between US$20 and $40 per person, these flights give you memories that live on long after you've forgotten the dent in your budget. For $55, you can fly by seaplane over Cape Grafton to Fitzroy Island, land in the water near Green Island, and pass over other scenic spots on a 45-minute trip. Flights in non-amphibious aircraft are less expensive.

Art collecting. Original paintings by Australian artists such as Joanne Hook and Spencer and Dianna Crooks cost less than you might think. If you can't afford an original, signed prints in limited editions start at $20.

Pedicab rides. When your feet rebel at going another step, a ride through residential areas lined with pink cassia trees and ``Queenslander'' houses built on stilts offers a charming change of pace at $16 for two.

Imaginative shopping. Rather than buying the usual souvenirs, try investing in a Canterbury polo shirt made in Australia, or items such as shorts, shirts, tea towels, tablecloths, and giftwrap designed by artist Ken Done.