Being on the computer forces your shoulders forward and together. A split keyboard helps bring your shoulders back into a natural position. I recommend the Kinesis Freestyle 2, but there are quite a few alternatives on the market like the Ultimate Hacking Keyboard and KeyMouse.
There’s a lot of research about how sitting all day is harmful to your health. E.g. a 2010 study in the American Journal of Epidemiology tracked 123,000 adults over 14 years and found that those who sat more than six hours per day had an 18 percent higher mortality rate than those who sat less than three hours. Treadmill desks are a really great way to combat this. The only one I’ve used is the Lifespan TR1200-DT3, which is one of the most popular and I’ve had a great experience with it. Walking on the treadmill can be a little noisy. It’s generally not bad, but at higher speeds I sometimes like to wear a pair of noise cancelling headphones.
Sit-stand desks work best with treadmills since they allow you to alternate between walking and sitting. iMovr makes several made to be used with treadmills such as the iMovR Everest. It has a tilting keyboard tray, which does feel much more natural and is very steady when using a keyboard. The downside is that it isn’t quite wide enough to be used with a split keyboard like the Kinesis. The other downside is that the mouse doesn’t fit on the keyboard tray, so is placed on the desk. This means it’s higher than keyboard, which is unnatural and places extra strain on shoulder. Overall it’s a good desk though and a titled mousepad can help angle the mouse at the same angle as the keyboard and make it more comfortable to use. You can even stack two on top of each other to increase the angle if necessary.