To gain a head-start in the beginning of a race, press and hold A just before the fourth light comes on and the race starts. If it works, you'll be boosted ahead of the pack, giving you a nice head-start for that race!
Get diamond in Cyber Field 2 (Grand Prix), at the end of the track in plain view
Vic Viper
Get diamond in Moon Road (Grand Prix); use Bear Tank and get a boost before the first jump, then as you hit the jump, use the boost and make a hard right to reach a secret island with the diamond
King
Get diamond in Sky Bridge 2 (Grand Prix); use Bear Tank and use turbo boost before first jump, staying to the left, then before the jump, make a sharp right as you jump so you reach a hidden island with the diamond
Ebisumaru
Unlock Bear Tank, then beat Time Attack records on Ganbare Dochu 1 and 2
8/14/2001
Control Replay Camera
After winning a race, or coming in the top three, you can control the replay camera and see the track from all sorts of angles. After your times and scores dissapear, press and hold the B button. This will lock the camera on your player. Now using the control pad, hit either up, down, left or right to rotate the camera or move it high obove your character. If you make the camera go as high as possible, you get a great view of the tracks you're on!