When my boys graduated from homeschool high school, I gave each a copy of "Oh The Places You'll Go" by Dr. Seuss. When each guest arrived at the party, they signed the book, right within the pages. We had some nice LONG comments, like a yearbook, and it was really fun. You can get the book to people who can't make it to the party as well, and that's a good way for them to "be there" when they can't actually be there.
The only really "planning ahead" thing that I did was make a set of photo albums for each boy. In years past, I've always gotten doubles when I had photos developed, so each boy had a shoe box full of photos. I added a few photos by making copies of the very best snapshots from when they were kids (that's not too expensive to just copy them on photo paper.) I put them all in albums before the party, for guests to look at. It was a big hit.
One warning, though, for all you scrapbookers! When I say I "put them all in albums" that was all I did! I bought photo albums at Costco, and slid the photos in. OK, so the pictures were roughly in chronological order, but that was really all I did with them. The guests liked looking at them as babies, etc. It was fun.