Sure is cheap here! Came with the ring!
I find playing with kids to be the cheapest, most readily availible (once you have one, sex has to wait till they are asleep), And most satisfying.
Dh 'rock and rolls' ()like crunching but more fun) with the kids (no room on my lap with a baby in the way

.) and he does 'kid presses' and all sorts of wierd things that encourage lots of giggles (laughter is great excersise!)
I have to say he is in great shape

, and no I am not just a biased wife, in a room of 30 something year old dads (specially 30 something year old computer geek dads) he stands out as trimest, most cut, and strongest if occasion to prove that arises. Yet he hasn't lifted a weight, ran a mile or anything else in ages (kids do get in the way of all that formal stuff.)
When not pregnant I found that chasing my kids keeps me slimmer and trimmer than when I worked out an hour a day.

The nursing and walking to the nearby construction stuff helps, as do 'baby presses' (sorry can't do the kids to heavy!).