Standing desks are hot right now (with various sources telling us that sitting is the new smoking). If you’d like to try one, the most customizable option is a motorized sit-stand desk. Karen Tkaczyk uses one, and she demonstrated it in this blog post. Another option is a VariDesk that sits on top of a standard desk.
I recently moved to a new co-working office and the desks have a really easy sit-stand option, where the desk surface is permanently set at standing height and you use a tall chair or stool, moving yourself (rather than the desk surface) when you want to change positions. Here’s my desk:

At first I was a bit skeptical about the stool, but it’s actually working out really well. It’s just uncomfortable enough that I get up for a few minutes every hour and go get some tea or run a quick errand; it’s not hellishly uncomfortable (and it forces me to sit up straight), but it’s also not the kind of thing you snuggle into for a four-hour stretch.
I think that my new desk is an Ikea Linnmon/Finnvard, which, at US $109, is certainly an affordable option. Just make sure to get a nice hard stool to go with it!