Tennis
You can choose the blue tennis court from practice mode as your tennis court in tournament mode. Enter tournament mode and select your character. Once the "Warning" screen appears, press and hold the "2" button on your Wii controller to unlock the blue tennis court.
Bowling
You can frighten the crowd in the Wii Sports bowling game by swinging the bowling ball back with your arm and releasing it before you begin to step forward. You can also get a laugh from the crowd by positioning yourself at the left or right gutter of your lane and throwing your bowling ball into the gutter of the lane next to yours.
Golf
According to GameSpot's GameFAQs website, you can disable the power meter, wind speed indicator and map in the Wii Sports golf game. Push and hold the "2" button at the course selection screen to disable these features.
Boxing
In the Wii Sports boxing game, you can earn silver boxing gloves by defeating Matt in a boxing match. To wear the silver boxing gloves, push and hold the "1" button while the black screen is being displayed. If you hear the bell ring, you have pushed the "1" button too late.