There are two things that I think are good. The first was one I didn't get to do because I didn't learn about it until too late. Southlake has a great place called Sunshine Glaze, and the owner is so helpful. She can help you make a plate. She even has the divided baby plates. You can put your baby's handprint, footprint, or picture and add whatever else to it that you want. Then they glaze it, and it makes a wonderful keepsake that yours can eat the cake from. That's what we did for the grandmothers for Christmas with footprints. FYI if you do that, you'll want to bring the baby in for footprints and come back another time by yourself to design the rest or have the employees make a design for a little extra cost.
What I did, though, and it turned out so well. . . I bought a wooden treasure type box from Michaels and painted it with my son's name and the year. I also painted sports stuff on it since that was his theme. Then I had little papers made out for guests to write on. One began, "I love you because. . ." Another "You have already impacted my life because. . ." And we had two others, but I can't remember what they were. We had them out by the box with several pens. Once they filled it out and signed it, they placed it in the box. It was really neat to read them, and it's something he can read when he gets older. You could have anything on the cards "You make me smile when you. . ." I think that was another one we had and maybe "One thing I wish for you is. . ." Good luck, and I hope some of these ideas help.