Can you find something on a wall that will support some weight? Like hooks drilled into wall studs (or some other kind of anchor) or perhaps the railing of a stairway? Exposed plumbing? If you can find a hardpoint on a wall somewhere, you can attach straps and a set of rings to it, and use that to do body rows with.
I recommend the body rows. ::EDIT:: because you can "game the system." They're easy to count, easy to time, easy to reproduce exactly, and easy change the difficulty of by changing where your feet are planted. This results in improvment that is easily VISIBLE which is pretty darn motivating. "See, I'm getting better! Wow, this really works!"
Now that I've lost an inch and a half of girth in the past month, and I can see my times improving slowly but steadily, I'm more pumped than ever.
This is a great workout program for a gamer/computer programmer.