Wii Fit Cheats - Wii

Unlock Challenge Mode (Muscle Workouts)
Get a 4 star on both reps in Press up and side stand , the Jackknife and parallel stretch to gain the challenge mode for this workout. Note: To gain the 2nd workout/ reps you need to do well in the first one so that the trainer tells you that you are doing well and should go for more reps/ seconds.

Jackknife challenge - Get a four star in both the 10 sec workout and the 20 sec workout to gain the Jacknife challenge.
Parrallel stretch challenge - Get a four star in both the 30 sec and 60 sec workout to earn the parrallel stretch challenge.
Pressup challenge - Get a four star on both the 6 reps and 10 reps mode to unlock the Pressup challenge.

Unlock Games
As a General rule to unlock new features play the previous games or complete the highest amount of reps or do more Body tests:

"Star stamp" - Attend 5 Body Tests.
"Ok stamp" - Attend 2 Body Tests.
Free Jogging - Do the Island lap 2-3 times.
Free step - Play Step Plus Aerobics 2-4 Times.
Longer Jogging Routes - Play the highest one to unlock the next one up 2-4 times.
Step Plus Aerobics - Play step Aerobics a 2-4 times.
Tightrope Advanced - Complete Normal Tightrope.
Tightrope Super Advanced - Complete Advanced Tightrope.

Unlock Alternate Jogging Routes
When jogging, occasionally a dog will run past you and your trainer. Quickly sprint past the trainer and you will follow the dog instead, taking you on a different route around the island.

Ulitmate Balance Test
There is the balance board character running on a treadmill in the background of the statistics screen. Click him with your pointer and you will be asked if you would like to participate in the ultimate balance test.

Unlock Stamps
Heart Stamp - Unlocked after 6 Body Tests.
OK Stamp - Unlocked after 2 Body Tests.
Star Stamp - Unlocked after 4 Body Tests.

Unlock Fitpiggy Colors
Bronze Fitpiggy - Accumulate 10 hours of training time.
Gold Fitpiggy - Accumulate 40 hours of training time.
Silver Fitpiggy - Accumulate 20 hours of training time.

Unlock Fast Mode
If you have time to kill but do not feel like going through the motions of unlocking new modes, you can choose the Balance Mode Games -> Marble Drop Game and deliberately fail the first challenge by letting time run out. Wii Fit rewards you one coin for every one minute of gameplay to unlock new game modes (win or lose). The Marble Drop game takes 30 seconds to fail, but the game rounds up your play time to the closest minute. This is a reasonably fast way to waste time (and press [A] to continue) if you're looking to unlock modes without earnestly putting effort into it.