The Arboretum offers a wide variety of classes and activities. Here's a sampling of opportunities over the next few weeks. The offerings are free unless otherwise noted. SPRING WILDFLOWER WALK -- Meet at the entrance to Fern Valley at 1 on April 22 for a guided wildflower tour. CRABAPPLE TOUR -- Meet at the entrance to the Crabapple Collection at 10:30 on Thursday for a walk-through with information on how to care for crabapple trees. ANNUAL SPRING SHOW OF BONSAI -- Running April 26 through April 28, it will include exhibitions, sales and demonstrations and will be open from 10 to 5. Admission is $2. WILDFLOWER IDENTIFICATION CLASS -- Beginning April 30, it includes an optional field trip to the Blue Ridge Mountains. Fee is $15. Fo registration information, call 475-4857. DOGWOOD WALK AND SLIDE LECTURE -- Meet at the Administration Building at 1 on April 30 for a walk through the dogwood collection; a slide show follows. CELEBRATION OF FERN VALLEY'S FIRST 25 YEARS -- On May 1 and May 5, there'll be demonstrations on wildflowers and ferns. On May 5 there'll also be a performance of Morris dancing around a Maypole in the meadow adjacent to Fern Valley. Programs start at 1:30 in the Administration Building. IKEBANA INTERNATIONAL FLOWER FESTIVAL -- Running from May 1 through 5, 10 to 5 daily, it will feature Japanese flower arranging and related arts; in the Administration Building. BONSAI CLASS FOR BEGINNERS -- A series of four classes begins May 7. The four two-hour sessions include a short history of bonsai plus instruction in branch pruning, root pruning, wiring and potting. Fee is $30. Call 475- 4857 to register. SPRING SALE OF HERB PLANTS -- Sponsored by the Herb Society of America and the Friends of the National Arboretum, it will take place in a tent near the National Herb Garden on May 8 from 10 to 3. IN PRAISE OF NATURE -- An art exhibit by Henry Wo Yue-Kee will be on view in the Administration Building through May 31.