Swimming is good. Anything low-impact, really. No long walks as a puppy, walking on sand, grass...less concrete and asphalt. Try to avoid lots of jumping until they're a bit older. I think I carried Discoe up and down steps until she was big enough where it was awkward and sometimes dangerous to do so...lol. Our back patio is only three wooden steps, but I would carry her up and down them until she was about four months old or so. Even after that, she didn't come down our basement stairs until she was over a year old.
Keeping your dog a bit lighter can also help. Not necessarily underweight, but a little under what your vet might see as ideal. A lot of vets operate on 'pet weight', which some working people might interpret as being slightly heavy (versus 'working weight'). A good balance on the lighter end of the scale is good, particularly if you aim to be more active with your dog.
Reba von Treue Freundschaft, CGC, TDI
SG2 CAC Discoe vom haus Huro, BH, AD, CD, CGC, HOT, OFA-Excellent, CERF