I've been using the ikea standing desk from a few weeks ago -- the lack side table with a shelf screwed to it. I simply put my laptop on top and then use the rest of the normal height desk as a sitting desk. I just move my laptop across to switch. I already had the standard desk and chair, so the whole setup cost me about £10.

I leave the keyboard, mouse and power adapter on the standing side, as motivation to spend less time sitting. So far I can manage alternate hours sitting and standing, which isn't too shabby considering how out of shape I am.