Secret Area:
On the sixth stage; Ghost Castle; go towards the rabbit instead of upstairs. Search the area for a crown and a locked door. If you have 50 crowns or more, you will be able to open the door and after entering be able to play a round of pool.

Secret Level:
Collect all the crowns from each level and defeat all the bosses. When you return to Stage Select you can now fight any of the bosses at any time.