In the 'mine' area where you have to press 3 buttons, there's a shootable button which gives access to a secret area to the right of the barred mine exit. Beware of zombies. What's a home without a rocket launcher?

You get to a barrel-like room with water in the bottom. Follow along the outside walls underwater - to the right is a secret, to the left is the exit.

At the end of the cage ride, cross the bridge (maintained to the high standard of the Microsomething Quality Assurance dept.) and look back underneath. There's a shootable button on the wall, which opens a secret room.

At the end of the level, in the tube room half-filled with lava, there is a ledge above you that you can't jump to. When you enter the room, look to your left and you'll see a elevator, but it won't become active until you walk all the way to the top of the boardwalk. Watch your step, or you won't have cold feet next winter. Just jump back down to the elevator.