Memento More-y!
I'm a community-theater geek (insert joke about "being dramatic" here). At the end of most shows, I've given a memento to the cast and crew. This might be an ornament (especially around holiday shows), knickknack, certificate, or something else thematically related to the show. If I were to run a longer-running tabletop game/campaign nowadays (especially with a group I like), I'd be tempted to the same at the end, as a souvenir of the game.
Some fairly quick-and-easy ideas:
• Candy! A game set in modern-day Japan or Europe would be a great reason to give a few regionally appropriate treats from the area they adventured in.
• Coins! Was it a modern (in the past 70 years or so) campaign? If so, you might be able to buy coins for the participants. For one production I was in, I bought 1950s quarters from the year the show first ran (it had a governmental subplot, so it made sense). If you're not super-concerned about condition, they're pretty cheap, especially if you're just getting a few.
• Pictures! A picture or illustration of a symbolic/interesting element from the game can be really quick and easy to make, especially if you have a printer capable of doing photos. There's even room on the back for the name and dates of the campaign, and maybe a heartfelt message if that's how you roll.
I could list dozens more here, but that should provide some starting inspiration. Regardless, giving players something to remember a beloved game can make tangible memories to be treasured long after the last session ends. If you commemorate your favorite games in some way, let us know on the forums.
-- Steven Marsh