April showers bring not only May flowers but also stir-crazy kids closeted indoors with nothing but tired, old (or so they say) toys and games as companions. You can keep them smiling, though, with some novelties that are affordable and are as far away as your corner mailbox. For starters, there's California artist Mona Allen Fisk's "Magic Puzzle," an ingenious puzzle that never bores because its surface is erasable. Color it (crayons are included), take it apart, piece it back together, erase it and start all over again. Specify 8 1/2" x11" ($3.95 or 10 1/4" x- 13 1/4 ($4.95, and whether you want large or small puzzle pieces; order from Creative Energy, 226 Colton St., Newport Beach, Calif. 92663. Another bonanza for dreary afternoons comes from the Smithsonian's traveling exhibition service, where education director Marjoire Share's drawing board is brimming with projects to get young viewers more involved in the Smithsonian's traveling exhibits. Among them are: Bright Ideas ($3.50), an educational board game for children 10 and older that celebrates the genius of Thomas Edison; Quilting Bee ($5.50), a kid that relates the history of quilting and outlines a child-size (ages 12 and up) quilting project using recycled fabric scraps from around the house or new material; The Big Circus Parade ($4), a miniature cardboard replica of the activities undet the Big Top designed for ages four and up; Carpenter's Lace ($3), a fold-up floor plan of a Victorian house with interchangeable decorations to experiment with, and a series of "architivity" cards to quiz children 10 and older on Victorian architecture. They're available at some Smithsonian museum shops or by mail. Make checks payable to the Smithsonian Institution (don't forget to add 10 percent for handling), and order from SITES, A&I, Room 2170, Washington, D.C. 20560.