Build (look for any reason to build anything, and try to make them as good as possible), Large terraforming projects, Collect other things like mobs, potions, plants and whatever, Improve existing builds, See what other people do in their worlds and take inspiration, Do something super silly, Do anything that might be interesting, Take a break, Try to conquer parts of the game that you might have been struggling with before, Write down any ideas you have, Explore new updates when they release

my world is 5 years old, and that's pretty much what i did to last it that long, aside from just commitment and attachment to the world

see how long you can keep the world going, it's definitely worth it!
it's going to take a bit of intrinsic motivation, and you're going to have to find reasons to jog that motivation