S.T.
In a Parent's magazine ( a recent one) I saw a cute activity that adults can do with children where you crack open an egg (empty it and rinse it out) and then keep the shell - try to get more than half the shell on the bottom to stay in tact. Put some potting soil inside of it and some grass seed and rest it in one of the cradles from the egg carton (recyling!). You can draw eyes on the shell and glue on a beak, or you can make a bunny face and add ears or whatever. It was cute - maybe you can find it online at the Parents Mag site.
You can always go and buy a flat of flowers from a nursery and break them apart into individual flower sections - put some spring colored tissue paper around the base, tie on a ribbon, and give a flower to each guest to take home and plant in their yard.
Can't wait to see what kinds of ideas you get from all the clever moms on this site!