I got an Ikea desk in the '90s that I've modified over the years: It's been painted a couple of times, and I used a sabre saw to cut out two of the shelves. Recently, wanting a standing desk, I rearranged the shelves and added 8" plastic bed risers from Bed, Bath, and Beyond. It's perfect now for standing. The mat is a $10 kitchen mat from Target. I don't always wear the shoes when I work, though I probably should: if you stand a lot, good shoes help prevent problems like swollen feet.
Because it's nice to go between standing and sitting, I bought a stool on clearance at Target.
You might think the stool would be too high for a cat to get into your lap, but Barnabas would prove you wrong.
Paranoia article of the day: Sitting All Day Is Worse For You Than You Might Think : NPR
Update: experimenting with a standing desk