What to do when your really bored on minecraft?

If you are playing the PC version, mod it.

There are thousands of mods for the PC version of Minecraft.

Some are just plain cheats, such as the numerous X-ray mods.,
Some are just silly, like a gun that shoots chickens.
Some add machines and other complex blocks, such as IC2, BuildCraft, and many more.
Some add more dangers, such as mutant mobs, better dungeons, and the Twilight Forest.
Some let you enter whole new worlds without ever leaving your game, like Mystcraft.
And some are so complete the make Minecraft into a different game, such as Terra Firma Craft.

There are many mod launchers out there, such as Feed the Beast, AT Launcher, and Tekkit that can automatically download and install whole groups of mods at once (known as "mod packs").