How about before all of the decorative lights come down; preferably prior to the 25th, where you spend a holiday-oriented weekend together.
There may be some relatively economical options at theme parks doing a holiday-oriented special (Hershey Park's christmas candylane is 20 bucks, for example). And it opens its doors for shops and coaster rides, etc, 'cept decked out with holiday lights. There's also free and cheap activities galore everywhere this time of year: just check your local paper.
Break out some ugly christmas sweaters/apparel and pack in a day full of holiday activity: sledding, baking cookies together, and setting up party favors/gifts for upcoming celebrations, board games, warm drinks, netflix, etc.
On christmas eve, if you live together, try to revive old childhood traditions like going to church or opening the stockings. This will help to restore previously lost magic buried under adult responsibilities. Visit old neighborhoods and reminisce. If the distances are drastically apart, visit yours next year and his the following, or vice versa.
I know you questioned us "what to buy" but I have been in poverty the last five years of my life and I know time spent with people is far more valuable than money spent. If I had a little and decided to buy them a gift, it would be one, innocent and non-fetish related during one of our holiday excursions, so when they hold it in their hands in the years to come it will hold a more cherished meaning.