Successfully complete a level with all treasure collected, all chests found, and all innocents saved to unlock an entry in the art gallery. Each level unlocks a new art gallery entry.
You can replay any level that you have previously completed as many times as desired. To get as much money as needed, replay an easy level, collect all the money, and save the game at the end. You will now be able to buy any of the upgrades that you need from the salesmen.
Sometimes when you see a breakable tree stump there will be a little rabbit nearby. If you hit it, it latches on to Maximo in a way that is reminiscent of the "Killer bunny" scene from Monty Python And The Holy Grail.