A couple of cardboard boxes* for the monitor (set on top of the existing desk return). One cardboard box and books stacked to get the ideal comfort level for the keyboard and mouse. A pretty throw over the whole thing, then add computer gear. Done, and everyone who walks into my office compliments it -- they have no idea it's a stack of boxes.
Since it's right on my return, I push the chair out of the way to stand and wheel it back over to sit.
* After about a day of wobbling I went back into the stack and stuck a couple of books inside each box to give the whole thing more stability.