My DH's aunt and uncle had a big family reunion for their 50th anniversary. They also stated no presents, so what I did was take tons of pictures at the celebration. Then when we got home, I made them a photo book from mypublisher.com (lots of places make them these days) of the reunion. They absolutely loved it and it wasn't that expensive to do.
For my parent's 40th anniversary I also made them a photo book, but that was very, very, very time-consuming as I had to scan in lots of old pictures, then edit them, etc. We had a friend burn the photos to a DVD and at their party we played it to Kenny Rogers "Through the Years". That was a big hit, too.
One other thing I did was make up some "happy anniversary" cards with their wedding photo on the front. I sent these along with self-addressed, stamped envelopes to all their old friends who couldn't be at the party and asked them to share a favorite memory. I took all the responses and mounted them in an album. My parents loved hearing from friends they haven't talked to in years.