The things that you can do are only limited by your imaginations, and also the game mechanics. Terraria is not just your regular action game, but it is also a game that lets you use your creativity to build stuff like castles, biomes, weapons, and many more things. The game offers so many things to do and explore, from many weapons, items, to fighting many simple enemies and hardcore bosses. Terraria is a very immersive game which when played once, will hook you to it.
If you are a fan of 2D action-adventure games then you will love Terraria even more because it is the previously mentioned element in a sandbox-style game.