That's a neat idea! Shapes of baby rattles or alphabet blocks would be a cool thing, too, if someone doesn't want to do leaves.
I went with leaves because my cousin (the dad) is a very outdoorsey type person. He works at a natural preserve on the weekends, bird watches for fun, that sort of thing. They also don't know the sex of the baby and had requested gender neutral items. Mom likes giraffes; so, I went wih their coloring of yellow, orange, and brown which happened to work very well with leaves.
I would totally do butterflies for a little girl, different kinds of trucks would be cute for a boy - it's totally open to whatever you'd like; but, there are all these sweet wishes for the baby, reminders to mom and dad to take it all in, and in a nice clutter free way. To be honest, it was kinda a cheat as I did the same sort of thing after my shower. I took all the cards and framed them in a big collage (sp?) and it's still in my sons room. Lot's of well wishes for baby.