You've had some great suggestions - but remember how quickly babies grow!
I'm a grandma multiple times, and that first year will whiz by. Be sure to get some cute "animal" or whimsical stamps for after the welcome newborn stage.
There are always lots of "girly" stamps around, but if you're having a grandson I think the choices are more limited. Papertrey just came out with a bunch of really cute little boy sets that are quite reasonably priced.
Boy Basics is really cute:
Papertrey Ink - Boy Basics: Truckin' It Mini Stamp Set
and Train tracks is adorable, too:
Papertrey Ink - Train Tracks Stamp Set