To change the controllers in the NES VC games, holddown the L + R buttons and press the y button. Repeat to switch back. Also, some NES games (such as Zelda 1 and 2) require you to press up and A on controller 2 to get to the save menu.
While playing a Game Boy game on the Virtual Console, hold the L+R triggers and press the Y button to change the colors from black and white to the classic green monochrome palette. Press the same buttons to return to black and white.
Once you get your 10 Play Coins for the day, go to "System Settings" under "Other Settings", then go to "Date And Time". Change the date to one day ahead, save the change, then go back to the current day. You can now get another 10 Play Coins. This trick will work multiple times in a day.