My favorite set of exercises when I really don't want to do any, is light weight squats and lateral lunges.
I use a smith machine for safety, put anywhere from a 10 lb plate to 45 lb plate on either side. Then do 4 sets of 20. While doing so I visualize the steepest, nastiest run I can, with a mandatory 10' air exit. To me it simulates the stresses on the muscle of an aggressive crud busting run.
Then I do lateral lunges. 3 sets of 10 - no weight. I concentrate on sinking low and then exploding off of the contracted leg, plant my imagninary pole, and lunge out again.
On days where I'm feeling more aggressive, I add a plyometric circuit, leg extensions, curls, abductors, adductors and calves.