Minecraft wiki entry for Things_to_do_when_bored, associated text: Try changing some of the rules! Here are some examples:
Level system:
Rookie (Beginning Level): You can't use tools or crafting.
New player (Levels 1-4): You can use only wooden and gold tools, only wood, planks, cobblestone, and dirt. You can't wear armor at the same time.
Introduction (Levels 5-8): You can only use wooden and gold tools. Only wood, planks, cobblestone, flower, dirt, coal, and gold can be used.
Citizen (Levels 9-12): You can use stone tools or redstone.
Knight (Levels 12-15): You can do anything a citizen can do, and use iron tools, and wear iron armor and gold armor.
Technician (Levels 16-19): You can do anything a knight can do, and you can create with diamonds and use diamond tools and armor.
Mage (Level 20+): You can do what any technician can and create with anything.