What favors seem to be used or make it home with their guests? I would say that in general they would fall into these categories.
1. Something with a little bit of practicality.
2. Food, if you have something that is prepared to avoid common food allergies and is so noted.
3. Something that has universal appeal. Take into account age and gender of your guests.
4. An object that has the bride and groom pictured in it. Perhaps they can use the frame for something else later.
Don't count on personalized napkins to be a favor! Who wants to take home a dirty napkin? No one that I know of! I'm not saying don't use personalized napkins. I think they add a nice touch. It might help people remember your anniversary date if nothing else!
If you have a centerpiece that is a live fish that someone will win at a particular table, that can get interesting. It's good if you're certain that one out of every 8 guests seated at a table is really going to want to go home with a fish. Please remember that if you do this, that you provide food and instructions on how to take care of the fish. If someone doesn't win the fish you want to still make sure that they have something to enjoy as a favor to take home. There are many creative ways to determine who wins the fish! It's also okay if someone doesn't want that little fishy and hands it off to someone who does.
Some people set up a candy buffet with containers for people to fill with candy and take home as a favor. I would suggest that you have some that are set up for diabetics! This will make them feel like they have taken into consideration rather than just another painful reminder that they can't enjoy candy or sweets anymore.
As always, wishing you the greatest of days!