A marriage record
I recently saw Sheri & Bob Stritof suggest making anniversary time capsules. I like that, but would suggest something more. Create an ongoing time capsule in the form of a scrapbook. Add things from important events, and select a few dates each year (anniversary, Christmas, each of your birthdays) in which you each write a “state of our union” letter.
Each time you add “state our union letters”, take time to read the letters from a year earlier, three years earlier, and five years earlier. Discuss the changes, good and bad. Complement each other on things that have improved. Reminisce and enjoy. Set goals for the next year.