@TriggerMeEmo I guess that depends on what your idea of fun is, and what you've already done. There was a recent thread in the Sims forums on this very subject, with answers from longtime players ranging from attempting a challenge to randomizing a sim's traits to just building or creating sims for a while.
A couple of other discussions might give you some ideas:
And if you enjoy achievement hunting, there are plenty of badges out there:
But if you're bored with TS3, maybe you'd benefit from walking away for a bit. I can go weeks without playing, and then I start to think of all the things it would be fun to do in-game, and I combine them into an idea for a sim or family. It's often more fun to come back to the game after a bit of a break, when things feel a bit more fresh.
Challenge 1 - Create a sim, pick an unfurnished lot, spend all your money making it look like a park, then see how your sim gets on as a 'tramp'. Mine ended up as a criminal mastermind!
Create six random sims, 3 male, 3 female use the cheat to give them loads of money, pick a house big enough add various extras, then leave them and see how things progress!