My 25th anniversary is next month, and I would enjoy any of the things that people have proposed.
Some other ideas:
A certificate that you will petsit so they can have a getaway.
Movie tickets, and if your budget allows, a certificate for ice cream for two.
Tickets to a play.
A board game (scrabble, cranium, etc.) and a certificate to bring takeout Chinese for four along with an evening of fun with friends (you).