I usually skip the gym (not that I am doing much now). And definitely, don't do anything at home that tires me too much.
Right now I am getting over the flu (was pretty bad on Friday). Yesterday I was much better so I did a bit of abs with my pilates ball (very soft, nothing exhausting). Today I have just been sitting on my balls lots and did some bouncing on it.
One thing I sometimes do is just tighten my abs and butt and then release, repeating several times (which I need to do, anyway, because of back trouble).