In my previous job, a few years ago, I used a standing desk rigged up to my normal desk by putting a coffee table on top. It looked ridiculous but worked well as I had a shoulder injury which was exasperated by using a mouse while sitting.
Anyway, shoulder is good but after using a standing desk the other day, I think they are great for keeping the legs and back active. I don't want to replace my desk nor do I want to put a coffee table on top of my desk. I've see these Varidesks which go on top of your existing desk but wondered if these were any good and if something can be found cheaper?
EDIT: Found this guy on eBay who rigged up his desk with a treadmill. LOL.
I use a cardboard box with a Fellows monitor stand propped on top of it. Keyboard and mouse go on top. Works fine when I have to be at a desk.