Mining or engineering. Mining lets you dig deeper into the crust of the world, while engineering lets you craft better mechanical items and lets you break the dungeon protectors. Oh yeah, you may also want to level up building too. That's' self explainatory.
Smasheroids are chief amongst them. Bring them to me and I'll buy :wink:, but other things like Autumns, Cornucopia's, Coffins, etc. are commonly found with high luck levels in mech chests. Red Chests are generally a crapshoot, but normally good for clothes
Bulbs can only be grown in purified temperate worlds, excluding the ghost tree bulb and cactus flower. Arctic worlds can be used for pine tree farms because placed compost has a chance to start growing a pine tree.