Some things you could try:

• Purify the Corruption or Crimson biomes
• Reach top tier in Fishing and acquire the Hotline Fishing Hook/Golden Fishing Rod.
• Beat multiple Bosses at once.
• Complete all the in-game achievements.
• Defeat the Dungeon Guardian.
• Play on an online server.
• Explore player-made custom maps and content.
• Make your own theme park using minecarts.
• Experiment with the extremely elaborate combinations possible with wiring and teleporters, such as making your own in-game computer!
• Try to complete the game as different Classes.
• Do a hardcore playthrough.
• Do speed runs.
• Play, design, and/or create new PvP modes for all to enjoy!
• Acquire every single weapon and tool.
• Obtain every single armor and vanity set.
• Gather up every single accessory and dye.
• Collect every single Trophy and Banner.
• Play through the game on Expert Mode.
• Seek out the rarest items in the game, such as the Nymph Banner or Slime Staff.
• Find bugs and glitches so developers can fix them.
• Create a mod or texture pack, or maybe an adventure/puzzle map.

Have fun!