Gaming on a Treadmill - What's Working
If you're going to attempt to play games on a treadmill, or while doing any form of exercise, there's a lot of things to consider when picking your game
Almost every exercise device requires some space at the front for handles/motors, so you're going to be further away from your screen than you normally would be.
Console games (and console ports, if you're using a PC) are designed to be played from a distance further than most PC games. The UI is designed big enough that you can read it clearly even at a distance when your head's moving around.
Music is so important if you're moving. A game with a great beat to it gets you pumped up and moving. If you want to play a game that's slower paced, try putting on your own tunes in the background.
Games with first person cameras just don't work. Not everyone has motion sickness issues with first person games, but the combination of "head bob" in the game and your REAL head bobbing at the same time can make you feel really terrible.
The ideomotor phenomenon is going to be your biggest problem though - looking at a moving first person camera, you're GOING to unconsciously lean. The more realistic the game, the stronger the effect is. This can be potentially dangerous if you're on an exercise machine.
In a game where you control the camera (usually third person) it's fine because you have a central point of focus.
In a game with a first person camera, the focus is moving relative to the screen, and your head is going to move, and it's going to affect your balance.
Games that require a lot of dexterity: MOBA's, shooters, and precision platformers, are going to be a lot harder. You're going to be playing in a 'distracted' state. Your reaction speed is slower, and your overall dexterity is going to suffer. Any game that's based more around rhythm or simple inputs is going to be more fun than one where you need to stop and figure out where your hands are on a keyboard.
Different consoles for different machines.
In general, when you're walking, you want your head to be upright - playing on a screen about the same height as your head is the ideal. If you're just walking slowly, a keyboard is fine but if you want to go faster, you'll need to use a controller. Your shoulders will need to move around, and your hands will come off the keys at some point.
A DS or a PSP is great for anything seated, where it's easier on your neck to be looking down for lengths of time. Things like exercise bikes or air walkers are great, but on a treadmill you'll probably end up with a sore neck.
I kind of hate to say it, but the Wii / Wii U can't be played on a treadmill/exercise machine. Aiming while walking is frustrating, and stopping everything to recalibrate the controller is just a pain.
I've talked before about this, but I thought Telltale's Back to the Future games were fantastic for playing on the treadmill, because of their "figure out a puzzle, get a cut scene" pacing. You knew the cut scenes were a few minutes long, and could literally put the controller down for a second and jog. You knew once the cut scene ended, you could take your time picking up the controller because being a puzzle game, nothing was going to kill you if you weren't ready.
Games that won't immediately kill you if you stop playing for a second are going to be better, especially if you're going to be interrupted - it's a lot harder to start/stop mid-game if you're moving.
Most importantly, PLAY THE GAMES YOU ENJOY. Making exercise fun is what'll have you going back to it on a regular basis.