We're going to have a little homemade photobooth. People can take pictures with the birthday girl, and it's a low-key, optional event without too much structure. Easy for kids and adults. I'll email the photos to everyone afterwards an they'll have a little keepsake.
I went to a 1st birthday party a few months ago and they just had food, cake, and presents and it was boring. Most of the time we were just sitting there making awkward conversation. I'm sure the no activity route works for some, but it can also fall flat.