Super Mario Sunshine is undoubtably one of the best games so far on Gamecube (Ninty's next-gen console), but in my opinion it just doesn't have the replayability that made Super Mario 64 so great. Yes, the idea is so innovative that only Shigero Miyamoto could have dreamed it up, but once you've finished the game, it's all over. As far as I could say, there is no extras at all.
Well, might as well get the job finished properly, then. This guide can help you to get all 120 stars, so you'll feel that there is no need for any extra modes, as you've already got the most out of the classic game.
Although, I have to say, there is no cheats for the game. This would give the game a bit more fun, e.g. big heads, unlimited water.
Despite this, the guide is a great help and is the best you can buy.