maybe have a random drop item called a "birthday candle" that you can turn into something special in a wizard shop or something for a free upgrade. Not sure if that's doable. Maybe instead have a very rare encounter of something like "birthday goblin/genie" or something like that. It's identical to a very weak monster in stats but when you kill it you get exactly 100,000 gold, effectively giving you a free present upgrade for a wizard shop.
Not sure if this is doable, but you could also have either birthday present post office quests that have really high rewards or have presents in the wild that drop good equipment either as being weird monster encounters or being items you could "open" when you win a fight?
Not sure what, if any of those are feasible for you, just some ideas.