A little hacking around will not hurt her at all. I've had LOTS of mares in foal and it will not hurt her. If she's used to daily lunging, that's o.k. too, but I would not recommend it if she isn't used to it. (as in all the time, not just occasionally) As I said before, whatever she is used to doing she can continue. A little less strenuous is o.k. too.
If you want to give her a a break when hacking around, ride her bareback. She'll appreciate not haveing the weight of the saddle or the tight girth.