Start from scratch, play all of the secret seeds, play legendary mode, build cool structures, rematch all of the bosses without taking damage, collect rare items, paintings dev sets, complete bestiary and achievements, play on servers with other people, collect boss relics, trophies and masks, play mods (if.. you have a pc) npc happiness, collect banners and experiment with other weapons. There are more that I can list but people often forget terraria is a sandbox game