This is a subject most runners I know like to avoid. Cross training is one of those activities that I really don't like. I know it is imperative to becoming a faster runner, but the whine wants to come out with "I don't wanna!". However, I force myself to partake at least once a week, sometimes twice. There is a class at my gym that is all free weights. I try to go every week, and it is great for my entire body. We do squats, lunges, and various lifts that target the triceps and biceps. As a runner, I find that my upper body is never as conditioned as I would like it to be. So, I force myself into this free weight torture. Other cross training I like to employ is the stair climber. It is a great workout for the backside and can really work up a sweat. I have also been known to participate in a spin class. I think the trick with cross training is to mix it up. Otherwise, the workout gets stagnant. I also add some core strengthening exercises, and that about rounds it out.
BTW, all this diet and exercise stuff seems to be working. I find that my clothes are getting too loose! Pretty soon I will have to partake is my favorite cross training activity: Shopping!