Have a "Silly Party" to celebrate April Fools Day. Call the girls prior to the party and tell them to wear whatever they have on at the moment (a "come as you are" idea). Have everyone bring something "different" to share as the food part of the party. Then, exchange presents - a totally useless/tacky item from around the house, wrapped beautifully. Play goofy games and sign silly songs.
Talk a walk and notice all the spring time changes. Don't forget to take a trash bag with you to pick up any trash you find along the way.
Celebrate Earth Day. Talk about recycling, cutting up six-pack rings, waste from fast food, etc. Bury different waste products (lettuce, apple, plastic, paper, Styrofoam). Plan to dig them up in six weeks and then again in six months.