If you're looking for a decent platformer to play on your iPhone, then Alto is a good choice. It essentially has you snowboarding through the alpine hills while rescuing runaway llamas and leaping over various obstacles.
The great thing about Alto is that it requires only one button to play. You simply tap on the screen to do tricks, while the snowboarder does the rest. It might seem too simple and boring, but it's a system that's tough to master.
In the end, it's a great game to play while trying to kill some time at work, and it's only $1.99.