We did a thumbprint project a couple of months ago with the kids from my moms' group, but I thought I would share the idea hand-out I used (click image for larger size). There are lots of examples of fun things to make out of thumb prints. All you need is some ink, paper, and markers. We had a broad range of participants, from toddlers to elementary school kids. Ed Emberley's works are the go-to for thumbprint ideas, but I found a few online as well. There are plenty more ideas to be had. Create a thumbprint zoo, garden, beehive, whatever!
Here's a beehive that we used (click image for larger). Kids could color the hive with markers, then put yellow thumbprints all around. An adult could add stripes and wings with a black marker to the "bees." This might be a nice gift for an expectant mom to decorate the nursery. Imagine if you had all the guests at a shower add their thumbprint and then present the busy hive in a frame to mom! What a nice keepsake. Another gift idea is for kids to create a thumbprint piece to adorn the nursery of a new sibling. It might be a great way for the other kids to feel helpful and not ignored in the commotion of preparing for a new baby.
I personally prefer bing.com to google when searching for examples. This is a great rainy day craft.