IN A CITY full of free museums and tourists' delights, Wolf Trap Farm Park's 15th summer season at the Theater-in-the- Woods and the Concert Shell may be still the best bargain.

Through August, you and your child get a day in the cool of the woods (and in the meadow, if you bring a picnic lunch) enjoying puppets, opera, theater, music and dance -- for free. All you have to do is make a reservation in advance for any weekday -- or for certain Saturdays -- and, on your arrival, present yourself to the employee checking names off a clipboard.

This summer you'll find some new faces as well as some veterans with new acts. Back again is the energetic, guitar-playing environmentalist, Billy B. (The B stands for Brennan, and why someone would drop a perfectly good name like that is beyond me.) Last summer, Billy managed to get stodgy adults, who thought they were merely accompanying their little ones, to stand up and act like trees and alligators. This is no small feat, considering that most grown-ups have somewhat rigid notions about the proper behavior that accompanies their stations in life.

But this is a man who, his mother says, "has never known a self-conscious moment in his life." Bill's father, a salesman, moved his brood of seven youngsters around New England and Wisconsin, where Bill -- the middle child -- presumably absorbed a love of the environment. He took up the guitar as a teenager and has managed to make his living presenting songs about the environment to people like you and me.

"I don't really do issues as much as I do the science of it," he says. "I go for the simple approach: 'What does this thing do?' those lines, he offers lyrics about flowers (they have seeds, not honey) and pollen, trees and photosynthesis, animals and, yes, even eat.

He is reportedly very popular at elementary schools, where he performs his own music and lyrics from three albums -- "Trees," "Romp in the Swamp" and "When My Shoes Are Loose." (All three are selling well, he says, and another album, "The Waltz of the World," is due next summer.)

This summer, Billy B plans to sing about changes in the seasons and present models of the Earth (pea-sized) and Sun (red and yellow and "furry"). Using his song, "The Animals Know," Billy B says he'll use breakdancing and other dance techniques to sing and talk about animals that migrate, those that hibernate and those that adapt. He plans to invite children from the audience to help.

Billy B appears at 10:30 a.m. weekdays at the Concert Shell in the meadow until July 26. Washington street peformer Bob Devlin, a singer of American folk music, takes over on July 29.

New this season to Wolf Trap is the Prince George's Opera Company's presentation of Humperdinck's "Hansel and Gretel," scheduled weekdays at 11 a.m. in the leafy Theater- in-the-Woods through July 26. The company's artistic director is Dorothy Biondi, who has been with the group since its beginnings 15 years ago. And although this production has been shortened from two hours to a tight 45 minutes, "Hansel and Gretel" is not new to the troupe, either -- the company has performed the Humperdinck opera at Christmas each year since 1978, and plans to take it into Prince George's schools next fall as part of the county's "Arts Alive" program.

No matter how other companies may have staged "Hansel and Gretel," this one is "completely our version," says Biondi. For one thing, not many operas involve audience participation. As narrator, Biondi will choose 20 children from each audience to serve as "frightening trees in the forest" and "gingerbread men who come to life when they are touched."

Biondi is especially pleased with the staging: Perhaps for the first time ever, a production will open the rear doors of the rustic Theater- in-the-Woods stage and use the natural trees as part of the scenery. The only artificial backdrop will be the Enchanted Cottage, used in the latter half of the show. Mary Pat Finucane and Carolene Winter star in the production as Hansel and Gretel, with Joan Bolling May as both the Mother and the Sandman, and Marguerita Kris as the "marvelously scary" witch, who smokes cigars, scratches and generally turns in a comic performance. Lamar Sims will provide piano accompaniment.

Noting that the cast has a complete backup of understudies (she calls it the "cover cast"), Biondi says that the Prince George's Opera Company recruits its performers from all over the Washington-Baltimore area, and tries to keep them busy "so they won't leave us and go up to audition in New York." WHAT'S ON IN THE WOODS

WEEKDAYS, JUNE 24-JULY 26 -- At the Theater-in-the- Woods: African Heritage Dancers (10 a.m.); Prince George's Opera Company's presentation of "Hansel and Gretel" (11 a.m); Blue Sky Puppets' presentation of "Rufus" (noon). At the Concert Shell: Billy B, guitarist and folk singer (10:30 a.m.).

WEEKDAYS JULY 29-AUGUST 23 -- At the Theater-in- the-Woods: Deaf Dimensions contemporary dance (10 a.m.); Bruce Hutton, a one-man band (11 a.m.); Library Theater's presentations of "The Boy Who Cried Wolf," "The Knee-High Man" (or "Little Big Horn") and "The King's New Clothes" (noon). At the Concert Shell: Bob Devlin, American folk music (10:30 a.m.).

SATURDAY IN THE PARK -- July 6, 13 and 20, at 11 a.m.: Offenbach's comic operetta "Fleurette." July 27, August 3 and 17, at 11 a.m.: Offenbach's comic operetta "The Island of Tulipatan." GETTING TICKETS: & THERE

Call Wolf Trap's Interpertive Office (255-1827) from 9 a.m.. to 3 p.m. weekdays for reservations. Special arrangements can be made to accommodate the handicapped. To reach Wolf Trap, take Route 7 west from the Capital Beltway at Tysons Corner (Exit 10W) for 21/2 miles and turn left at Towlston Road, at the brown Wolf Trap directional sign. (The Dulles Toll Road is not open for Wolf Trap's daytime programs.)