Monthly picture on one side, text on the other about the month. Take it a step further by taking their picture with the same thing every month so you can see how they've grow and change. Maybe continuation of pregnancy book
Favorite Part of this is to make a stepping stone path. HOW: cup salt, cup flour, cup (give or take) water. Knead until dough forms. Make impression. Bake at 200 for 3 hours. Do every summer and make a stepping stone path :) What a cute idea!