To get to the casino, go to Bean Valley. In the area where all the pipes are located and the Shy-Away is watering the Piranha Plants (not the area where you fight Smilax), wait for him to water the one closest to the save block. Beat the plant and go down the pipe. Down here, in I believe the northwestern corner or somewhere near there, you can jump to find an invisible platform and reach Grate Guy's casino. You'll need the card to get in. You can play blackjack, a ? box slot machine, and "look the other way." If you win look the other way a lot, you'll get the Star Egg. I've never gotten the Star Egg myself, but from what I hear it's a reusable Rock Candy.