The eight worlds are divided into eight levels apiece, topped off with a boss battle. Given how hard it is just to get around in monkey world, none of the bosses can offer much in the way of memorable confrontations. They all have some kind of glowing weak spot you have to jump and hit, each with a predictable pattern and re-used attacks. Granted, they have some unique bit to them (bounce missiles back at one, systematically dismantle another), but they're not gonna make or break your decision to play the game again and again.
That honor belongs to the 50 minigames ready to go from day one. Wii has already become the minigame console king, and it's Banana Blitz that cements that status. Homerun derbies, treasure hunts, UFO attacks, snowboarding... if you can imagine monkeys doing it, it's probably in here. The catch is that only 10 of them are cool enough to play over and over (hammer toss). 10 morepretty muchsuck hard (trumpet blowing?) and 30are fun for at least acouple of tries (slingshot shooting).
