Napa Valley. As mellow as a sunny day, with the savor of luxury and the spice of adventure.

FLOAT over it. Ride its trails. Relax in its spas, savor its renowned cuisine, and be pampered in its lovely inns. California's Napa Valley is a tourist's paradise. Within a four-block stretch of Calistoga's main street, you can rent a bicycle, soar in a glider, play golf or tennis, try out a spa, browse in antiques shops, and ride in a hot-air balloon. But spread the delights over several days for a refreshing interlude during a San Francisco visit or for a holiday of firsts. Ballooning over Golden Gate

Once in a Lifetime Balloon Company is one of six companies floating their eight-story-high, rainbow-hued balloons over the valley early each morning, weather permitting.

Liftoff is silky smooth, quiet, magical. Aloft you glimpse the Golden Gate Bridge and the shimmering Pacific Ocean 60 miles to the southwest, as you drift peacefully through an azure sky. This first costs $140 per person, including a gourmet brunch.

Pilot Bob Allen says, ``Drape a silk scarf over the rim of the basket. Not a flutter. We travel with the breeze. Even during a race, when altitude winds move us at 70 m.p.h., a candle will not blow out.''

A chase car brings us back to Calistoga's main street and the Mount View Hotel, vintage 1917, grandly restored in 1980 to its original art deco style. two blocks down the street Jules Culver will fit you out with a 3-, 5-, or 10-speed bike for $6 an hour, $9 for two hours. We liked the ride out to the fairgrounds and through the vineyards, sampling ripe zinfandel grapes. A bath in volcanic ash

By early afternoon it can be time for an excursion to one of several spas in the area. What do you find there? At a typical one we found a mud bath, a thick soup of volcanic ash, or hot mineral spring water and peat moss. Some people think it improves their health or beauty. Others try it just for the adventure.

``Don't be squeamish,'' the attendant says to the mud bathers.

Afterwards there's a whirlpool soak, a hot-towel body wrap, and a massage. The cost: $44 for the works.

Visitors to Napa Valley have a choice of six motel spas with a variety of offerings. Gliding over Calistoga

How about a glider ride into the wild blue yonder?

Jim Indrebo, teacher at the Calistoga Soaring Center, has been instructing students and piloting visitors above the valley's vineyards, forested hills, and ranches for 12 years. In the three-place Sweitzer sailplane, we buckle down into the seat behind Mr. Indrebo. At 2,800 feet the Piper tow plane lets go, and we ride serenely on the air currents, banking around the mountains and thrilling to the thermal lifts. The cost: $55 for one person, $65 for two for a 30-minute flight. An unforgettable first. Memorable dining in two towns

The Napa Valley offers an array of superb restaurants, from modest to posh. Before leaving Calistoga to drive down the valley on our rambling inn-hopping tour, we lunch with Mark Dierkhising, owner and chef of the All Seasons Cafe. His restaurant, like a miniature French market, sells rich homemade ice cream, heavenly pastries, and picnic items to go, as well as its own smoked fish, meats, and poultry.

This chef believes he's working at an art form. We heartily agree when plates of colorful appetizers, dainty shish kebabs, and paper-thin slices of smoked salmon with baby vegetables are placed on our table. Each is picture perfect, exquisitely seasoned, light and satisfying.

In St. Helena, we dine at the top-rated but unsnobbish Le Rhone. Mme. Chalaye explains that the first course, ``Chef Surprise,'' is her husband's innovation, always concocted from the freshest fare in the market that day. This evening it is sea bass with mussels in a savory sauce. Hors d'oeuvres follow (salad of sweetbreads with peppers), then spinach pasta with scallops and the new California rage, golden caviar, from local whitefish. For dessert, ask for a sampler plate of pastries.

We have a leisurely dinner at Pasta Prego, in the Grape Yard Center in Napa. This Italian-style trattoria is run by chefs Charles Holmes and Gregory Cole, and their antipasto, salads, and entrees range from exotic to classic. Inns for varied tastes

Gallery Osgood Bed-and-Breakfast on First Street in Napa is the affectionately preserved 1898 Queen Anne-style home of Joan and Howard Osgood-Moehrke. A gourmet cook with a library of 400 cookbooks, Joan serves fresh fruit plates that are as striking as her silk-screen prints. She keeps a card file so that returning guests never have the same sumptuous breakfast twice. Rooms are $75 (double occupancy), with semiprivate bath.

Bartles Ranch, three miles out of St. Helena, is a sprawling stone and redwood ranch house set on 100 acres of rolling hills, pines, and centuries-old oaks, with horses grazing in the meadows.

Jami Bartels has turned three large bedrooms in her family home into Hollywood creations. In the ``tropical sunset room'' you sleep on a king-size waterbed under a fishnet canopy dangling seashells. Your bath is a sable brown, gold and silver art nouveau fantasy. You can step out of your bedroom onto the deck and enjoy a Jacuzzi under the stars. Cost is $90 to $140, with continental breakfast.

Quaint or trendy, B&B d'ecor is sometimes overdone. Often it's the interior d'ecor that clamors for attention. Not at Villa St. Helena, however. Commanding a panorama of the whole valley, Ralph and Carolyn Cotton's Tuscan villa is the epitome of understated elegance. Heirloom French provincial furniture suits these nobly proportioned rooms, as do the simple white eyelet curtains, which never compete with the view. The gracious 1941 house curves around an inner court. Each of the 10 guest bedrooms opens onto its own balcony or onto the swimming pool veranda.

Rates are $85 to $185, with continental breakfast. Sightseeing from horseback

Horseback riding in Napa? The trails at Wild Horse Valley Ranch take us past the Western training site for the United States Equestrian Team and the Olympic cross country, steeplechase, and show jumping courses. On 3,000 acres, every kind of rider can enjoy breakfast or dinner rides, and instruction in competitive riding, polo, and dressage. Wrangler Ren'ee Celler tailors a two-hour ride-cum-sightseeing for us ($25 per person, $30 for a breakfast-at-the-lake ride).

Except for January and February, when the rains come, Napa Valley is blessed with warm, sunny weather. For accommodations in any of the valley's 60 inns, you can phone B&B Reservations (707) 224-4667, (707) 252-1985, or (707) 257-7757. Rates run from $35 to $410 per night, double occupancy. Reduced rates midweek and between November and March.