Secret levelsOn the PC, neither Bubble Bobble nor Rainbow Islands contain the secret levels found in the arcade versions.
Treasure roomsIn levels 20, 30 and 40, you will get an item that transports you to one of three treasure rooms -- if neither of the two players lost a life up to then. Reaching the first chamber is easy with some experience, no. 2 is a lot harder, and the final one is virtually impossible to get to. If you're one hell of a player and reach level 50 without life loss, a secret door transports you straight to level 70.
Each treasure room features a message written in code. The first room text is "IF YOU WANT TO BECOME THE OLD FIGURE, USE THE POWER OF YOUR FRIENDSHIP, AND FIGHT WITH ME!", which means that you should fight the end boss with two players.
Room 2 reads "I ENCLOSED THESE MAGICAL MEDICINE HERE, BECAUSE THOSE ARE MY WORST FEARS" and hints to the way the end boss can be beat.
The last message is "IF YOU WANT TO GET BACK YOUR LOVE OF TRUTH, YOU MUST HELP EACH OTHER UNTIL THE LAST", which most likely again refers to using two players.