Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...
Unlock the Banana Temple Beating the game unlocks the Banana Temple, but it won't be accessible unless you do a few things first. To gain access into the temple, collect the K-O-N-G letters in every world and get the orb in each of the world's temples. Once you've collected all the orbs, the door to the Banana Temple will open.
Beat every "-K" level (get K O N G on every level in a world) and beat the game. When you do, a golden temple will show up on the world map. Beat the level there and you'll unlock "Mirror mode," a very difficult way to play through each level, in which Diddy can't be used, you only get one life, the level is reversed, and you can't use items.
Extra lives – Low Battery Special
Resume a saved game with a Wii-mote that has a low battery. Cranky will warn you that your battery is low and give you three extra lives. He’ll give you 5 extra lives for no reason sometimes.
All levels in a world except Boss levels have hidden K, O, N, and G letters. Collect all four of those letters in all levels in a world to unlock a bonus level. There are a total of eight bonus levels.
Infinite DK Coins
In Level 2 – 2 ( Sloppy Sands) there is an area before the checkpoint pig with two squid turrets below a bridge. Use Diddy Kong’s jetpack to super bounce from the lower squid to the higher squid. Hit the squid and drop to the lower squid again. Keep bouncing between them for infinite DK Coins.
World 9 – The Golden Temple
There are 8 Rare Orbs to collect, one in the bonus level of each World. Once they are all collected, the path to World 9 via The Golden Temple will be unlocked.
Successfully complete the bonus level in a world to get a Rare Orb.
Using Roll Recovery to get KONG letters
When there is a KONG letter hanging off the edge of a cliff, use the Roll Recovery move to get it. Roll off the edge of the cliff, and then jump at the last second to reach a safe ledge above. Note: This requires precise timing and is not easy to master, but it is the only way to get some collectibles.