A site with lots of good info about standing desks and a variety of products and accessories: http://notsitting.com/, some nice desk options like Lynn described and a bunch of others, for anyone who is not doing the rigged up method.
Rather than an under-desk treadmill, I picked up a Gold’s Gym mini-stepper for $20 on Craigslist, works perfectly for what I needed.
This weekend I plan to ditch the boxes that support my stand-up desk, in favor of the simple pipes for leg extensions like Ron’s been telling us about! Skipping PVC for mine though. Black steel pipe is not much more.
Another piece of this puzzle for me is remote control software e.g. TeamViewer that I’ve been trying out, so when I switch to or from my standup desk I have the same thing on my dual monitor displays at my sit-down desk as well. Works great, especially when I realized you can pop each monitor out in it’s own window at the other computer and full-screen them, and switch to either side in the software quite easily. I’m running about the same software on each computer, though I can try a new version out on just one of them and have the other to fall back on. Oh wait, we’re not talking about computers in this thread…LOL. But, it is good for your physical health to not do one thing all day - especially if that thing is sitting, and it is good for your mental health to not be tied to only one set of solutions because they inevitably fail sometime.
The site at the link above also has tall desk chairs, or desks easily adjustable like Lynn’s, so it is not necessary to run two machines, that’s just working well for me as an option because I decided to totally wipe an old laptop and rebuild it, which ended up faster than my new laptop.