The idea: Have everyone write a letter before (or at!) the shower to baby on their 18th birthday. It might feel a little silly projecting so far into the future, but at a very pivotal time in their young life, you'll be giving them a gift from the past from all of the adults who love them, past and present.
Make it extra meaningful: Slip photos into the envelope with the letter so that they can see "old" photos along with your note. Or, take photos or polaroids of each guest at the actual shower and slide them into the corresponding envelopes before sealing them.