Terraria [official site] has plenty to do already. Have you ever sat and just fiddled with it for a few hours? Interestingly enough, it also utilizes the same font I used in every report I turned in to my Language Arts teacher in middle school (it used to be called Mead Bold back then, but now it's called Andy). So you're forgiven if you feel a bit overwhelmed when you look over the Super Terraria World mod, which converts vanilla Terraria into an RPG with quests, NPCs and more.
Re-Logic's classic sandbox game has been reborn thanks to one ridiculously crafty dev team you can interact with over at the Terraria forums. There's a staggering amount of stuff that's been added. So much, in fact, that if you haven't picked up Terraria and explored in a while, this mod might change your mind. It's lain dormant in favor of my visual novel addiction and Princess Maker 2 Refine lately, but I'm strongly considering returning to take in all of the new content.
Character skills, passive abilities, and a story to explore to your heart's content means you should have tons of new things to do. The features you already know and love should still be there, too, but they've been given various improvements.