You can do a spring theme like grass/hay/alfalfa/moss (real or fake), dew drop beads (those little liquid balls they have at the dollar store for flower arrangements), smooth pebbles, cotton clouds (or cotton candy), feathers/ faux fur, or pastel colored fabrics with different textures. also look at the birthstones- for April its diamonds so maybe you can get some fake diamonds and hide them in a sand box (kinda like an archeology dig) or something bejewled with fake diamonds (or rhinestones).
Maybe you can do something like something like april fools too (maybe a whoopy cushion or something)
Dew drop beads (they come in different colors):
http://www.google.com/imgres?q=clear...0&tx=130&ty=65
Birthstone chart:
http://www.timewellcorp.com/birthstones.htm