Bowser Jr, Boo, Shadow Mario, and Petey Piranha are the secret characters.
Here is a list of cameos: Toad, Chain Chomp, Shy Guy, Bullet Bill, Bob-omb, Piranha Plant, Para-troopa, Lakitu, Thwomp, King Bob-omb, and Whomp...and Bowser's Koopa Klown Car. There is also a statue of Toadsworth in the Mushroom Kingdom level. Let me know if I left anything out.
The only complaint I have with the game is that Bowser Jr IS Shadow Mario in Mario Sunshine...so how is it possible to play as both characters?! It's just like Camelot's screw-up with the first Mario Golf where you can be Mario, Metal Mario, and Baby Mario. Oh well. I agree it would have been better to have Toad or Toadsworth as a unlockable extra (instead of Shadow Mario). And since Rare was sold off to Microsoft, Diddy Kong became an official Mario character...and that is why he is in Golf TT and in Kart Double Dash.