When doing thank you's in bulk it's always best to a simple design, and in many ways less can be more elegant. Below is the picture the cards I made for my baby shower I had recently. I chose a Peter Pan theme for the nursery, one it's not over done, and two it works for a boy or girl.
Since I did find out we are having a baby girl I focused more on Wendy and Tinkerbell.
The thank you cards I made were quite simple as I found green cards (Crafts2000) and I have the Disney Cricut cartridge. I cut out the shadow of Wendy in size 2" and placed it in the center. I didn't glue the picture down, but instead used the 3D stickers to give some dimension to the card. Then I added self adhesive pearl accents in each corner. On the inside I did stamp Thank You leaving enough room to write a personal message.
VOILA! Done.