It's not actually all it's cracked up to be, unless the tub is large enough to fit two people, which they almost never are. You can do it standing up, either from behind or face-to-face, but you won't be doing anything on your back or knees, unless you're like three feet tall. If you want a little variety, there's always the kitchen table and the couch. If you're in a shared living situation, a car offers all kinds of interesting variety
. Just make sure it's not moving.