Depends on how "under the weather" I feel. Sometimes, it's best to not workout.
But, if I do, then yoga (just active enough to get a sweat going), or some low- or moderate-intensity cardio (about the equivalent of going for a walk).
I've also done short "arms" workouts just to keep them toned and to do something: a pyramid set of bicep curls, overhead presses, triceps kickbacks or standing extensions, first with a light weight and higher reps (20-24), then with a heavier weight and fewer reps (18), then the heaviest weight and fewer reps (15), then going back down the pyramid.