If I was moving away from a close knit group, I would appreciate photos of each couple in the group, and depending on how close, a photo of each couple’s family.
Even if it is just candid photos taken at the party, it will mean so much as the years go by to capture these important friendships.
An easy thing to do is say No Cards, but have 3 x 5 or 4 x 6 blank cards at the party for people to write a personal note on. Easier to compile and save in a slip in photo album later, along with photos of the party food, cake and guests.
To take it further, if you have time, take some photos of their current house, neighborhood pool or other local spots, and other places your group has hung out over the years.
You can digitize the album to make it less bulky, if digital is more your style, and then make a copy for yourself as well.