Here are some cheats/Glitches i found while fooling around.

Glitch 1:Under Delfino Plaza: get turbo nozzle.Go to the crate near the "Teleporter" to Noki bay.now go in the water under the crate.Turbo nozzle right to the wall. mario shouldnt "BUMP" into it.go right under the crate and JUMP!sometimes youll be in the crate,so head for the wall.Wen youre under,Keep jumping or die!

Glitch 2 "Inside" a boat:Whatever way, Get under water walking.Go to a boat.go behind it,pushing a "Wall". At some point,ull be under it.now get inthe middle and BACKFLIP (Or SPIN JUMP!) and youll be in the crates on the boat.

Glitch 3:Inside a durian (you know those kickable things? thats a durian)
ok this is probably the easiest glitch EVER! to get inside a durian, simply ground pound it.OR kick it to a corner and run at the wall.(Keep Running!) then it will go insideyou,spinning.

Glitch 4:The Turbo Nozzle Spray:Ok this is not so hard.have turbo nozzle.switch to spray nozzle.hold R button.keep holding it,andstill holding R press x. you should be spraying with turbo nozzle!

Glitch 5:I call it "Halfway there".
this is yet another easy glitch.Get Turbo nozzle. Go to Gelato beach.once there go to the spot near ricco harbor you cant pass. ok now use turbo nozzle.you will break thru this wall, but then theres a wall that u REALLY cant pass.This works at some other spots too

Glitch 6:Under Pianta village:
Go to any nighttime episode in PiantA VILLAGE.now go to the end of the river.there will be water still going out the end.hover to it.you should be "Swimming". now press B twice(or was it three times??) you should safely make it under Pianta Village.Ps the only way out is to press start and exit.

Enjoy!