My BEST friend at the end of a long day - being on the bike and on my feet lecturing since 9.30am this morning:
Yes, a tennis ball. And a yoga mat (if you are as disgusted and childish about dirty/filthy floors as I am... I refuse to lie directly on the floor...even in my own apartment!...).
Anyway, place the tennis ball underneath whichever body part that´s aching and lie on top of it. Yes, with your full body weight. It may hurt as hell, but it´s paradise afterwards...