Spoiler:

The first secret exit is in the beach stage. Between 2 pipes that have upside down piranha plants, use Raccoon Luigi to fly up into the bricked area, being sure not to hit the brick almost directly beneath so you can enter the side pipe. Inside the bonus area is a Tanooki Suit. Grab it and head over back to the beginning of the stage. Turn into a thwimp while over the "eye" blocks and you should bust them open. Go down the pipe and beat the boom boom to create a bridge to the secret stage in the middle of the map.

After passing the Thwomp puzzle, make sure you have Raccoon Luigi. Get a running start and start flying before you reach the fence. Fly to the platform that leads tot he lava and fly up through the ceiling inside the first lava-fall and hold right. You'll go through the wall and continue going right to get 2 prizes and a slightly different boss fighting arena

The alternate secret area needs Hammer Luigi. Just before the boss arena door, go into the lava (remember, Hammer Luigi can do this!) and go direction to the right, straight through the wall. You'll get a star before going in th door and the ? block on the other side has another star in it if you hit it before running out of your original star, making the boss a gimme fight