After you get the first three princesses your castle will be upgraded again. At this point it will include a table tennis table in the basement area. Examine it and it will ask you if you want to play. Each difficulty you beat your opponent on unlocks a different item. Also each difficulty you beat unlocks the next difficulty.

Item	How to unlock
"Amateur" Difficulty	Beat "Rookie" difficulty.
"Pro" Difficulty	Beat "Amateur" difficulty.
"World Pro" Difficulty	Beat "Pro" Difficulty.
Bamboo Sword	Beat "Amateur" Difficulty.
Dried Squid	Beat "Pro" Difficulty.
Legendary Undies	Beat "World Pro" Difficulty.
Newest Televideo	Beat "Rookie" Difficulty.