(1) SUPER SHORTCUT!! This is a really cool shortcut. Although a turbo is helpful, you in fact, don't need one. Just go up the ramp and at the tip, make a little jump with the "R" button. You can also use a turbo to boost yourself into the tunnel. Either way, you should fly right into the tunnel and exit through a waterfall.
(2) Another shortcut is across the sandbar. After you pass the huge rock go to the right. Just be aware of the crabs or you'll spin out.
(3) On top of this grass hill, you'll find an item box which contains a Spiny Shell! To get it, just go up the ramp and aim for the box.
(4) Try to Power Slide around these turns.