Special spells
Go to the alley right next to Toad's house in Dry Dry Outpost. Jump forward over the left box and the camera will swing around to reveal a hidden area. Talk to Merlee to learn about her special spells. Depending on how much money you spend, she will help you during and after battles. The spells do wear off after a certain amount of time, but until then, it provides you with additional attack power during some battles and more coins after battles.

Hidden playroom
Go south in Toad Town until you get to the area with the Li'l Oink Farm. Go left immediately after you enter this area (there is a narrow passageway that's not fenced off). Hit the tree with your hammer and a pipe will appear on the right. You can now enter the playroom, but need either a Silver and/or Gold card to play the games.

Classic Mario music
Go to the third house in Koopa Village (a Koopa is sunning himself inside). Stand in front of the radio and press A to switch between stations. Listen to the Golden Oldies station to hear classic Mario theme music.