• Tribal Island Trail(1)
Just before you come to the lake with the crocodiles in, drive behind this pillar to cut out the corner.

It's okay, you do get a power plus brick out of it, but its a bit slow and hard to manage.

 

• Tribal Island Trail
Just after the bend following the crocodiles, you come to these two pillars. Turn directly left (circuit 2) behind the left pillar or directly right (circuit 5) behind the behind the right pillar to find another shortcut.

This is an 'okay' shortcut. You get several power up bricks and cut out a long stretch but sometimes you can crash trying to get into and end up last.

• Royal Knights Raceway
Just after the interior of the castle (at the start), turn the corner and then turn behind the right tower (circuit 2) or the left tower (circuit 5) to cut out the corner.

It isn't the greatest shortcut as it is on grass but you get a power plus brick and it does cut out a large corner.

 

• Ice Planet Pathway
After crossing the iced over lake, take this shortcut (on the opposite side on circuit 5) to cut out the corner.

A pretty good shortcut with three power plus bricks in it. Quite obvious though.

 

• Amazon Adventure Alley
At the very start of the race. Drive through the waterfall for a shortcut into the temple.

Not that bad, but it takes time to drive through the water. Remember to collect the Power Plus brick on the way through.