I just found a Keepsake Box that I kinda like. I have a baby book, a two shutterfly photo books from my son's first year, and a keepsake box with all of those special elements in it. I've been thinking about making a Shadow Box and putting his delivery stuff in it, too, but I dunno. I kinda like this idea from Spoonful better. Everything's all packed up and ready to go, ready for them to look through it later on; it's labeled and organized. And I can store things like this idea in it too (only instead of writing a letter to them, our kids are going to get a box of 18 birthday cards that each have $10-20 in with a note from us).
Ooo, and here's another good idea for shower cards: turn em into art:
Veronika's Blushing: DIY Transform Your Baby Shower Cards into Nursery Art!