Most of the feathers are pretty easy to find and you can't miss them, but if you miss some you can go to level select to see where you need to go back to.

Drive around all 3 laps on each race track. No need to worry about the ghost car driving around, just finish the races.
Go to the corner where there's a bunch of books stacked on top of each other and jump up to the microwave to find the cassette tape.
Throw 15 paper planes into the trash bins before the timer runs out.