There aren't many legitimate cheats for the game, but the main one allows you to access characters and challenges which were time-specific i.e. they should no longer be available.
You can see this cheat below:
Time Change Cheat
This cheat allows you to gain access to all of the limited characters, boards and missions in Subway Surfers. To do it, access your device's settings, go to Date and Time, then change it to whichever time period corresponds to the character, challenge or board you wish. You can see these below:
Zombie Jake (character)
October 3, 2012
Elf Tricky (character)
December 1, 2012
January 5, 2013
January 30, 2013
Februrary 28, 2013
Egg Hunt/Chicky Hoverboard (Challenge/Board)
March 1, 2013
This cheat is quite hard to implement, but, as well as picking up the Coin doubler multiplier, you can earn twice as many Coins if you have the Super Sneakers and Jetpack active at the same time. You should upgrade the Super Sneakers to make this easier to occur. It won't make a tremendous difference to your score, but it's nice all the same.
Feel free to add your own cheats to the comments below!
The first thing to say about Cheats in Subway Surfers is that there are many hacks out there which can be downloaded to break the game - allowing you to get infinite Coins, become invincible, and so on. Or at least this is what they purport to do. Many of these are in fact viruses or people trying to gain access to your device accounts.