I got some cheap pine from Lowe's and attached it to an old desk surface I had in the garage with clunky cross braces cut from the same pine. I basically just made a long rectangle for each leg and screwed them to the surface. I propped it up on old yearbooks when I realized it was a little short. It's a little ghetto, but it works. Monitors are cheap these days; $150 will get you a decent 24" screen.
I have a home gym with a treadmill as well and made a desktop for it for under $40. I followed these directions, basically: http://www.dowerchin.com/2012/01/23/diy-treadmill-desk-under-50-no-tools-required/
I have an old PC hooked up to a 50" LCD in the home gym. You could just stick a laptop on the tread-desk if you wanted, or just read a book while you walk/jog/run on it. I have a keyboard and mouse that I can use fine at 1-2mph, but over that and I just watch video because typing and mousing gets a little difficult.
On a typical day, I'll switch on and off between the standing and sitting desks every hour or so, and hit the treadmill once or twice during the day for like an hour or two. I really should use the treadmill more.