At any time, type any of the following: (screen will flash to let you know if it worked.)
"NORMAL"-----Disables all active cheats
"LIFE IS TOO SHORT"--For infinite lives
"IRON MAN THOMAS"-----For invincibility
"ANY LEVEL"---------Activate stage skip
(Down cursor to advance level and up to go back)
This next cheat didn't work on my version of the game, but I left it in just in case it works on other versions. At any time, type "TIMEFLIESLIKEABANANA" for infinite time.
There is a secret room located on level 1.3, and can be found by climbing up to the ninth branch of the tree (which is the one with the birds on it) and running off the end. You then will be whisked off to a room containing some diamonds which can be collected for extra points.