Most Helpful Girl
It can be anything. You can give him a small, thoughtful gift (something like an inside joke or something). Something else you could do is when he wakes up one morning, surprise him with breakfast in bed (including for example self-made croissants, some nice fruits, a smoothie, candles or a small card with a sweet message). What you can do as well is doing something together that he likes. So if he loves gaming, while you don't, maybe plan a date where you get some nice food and then play games together.
Baking/cooking together is also fun!