I just realized I hardly answered the question.
I did however touch on an important point, and will emphasize again: low impact weight bearing exercises.
Walking, hiking, stepping- these are all low impact and help maintain if not promote bone density.
Most weight-bearing exercises are great as long as they are low-impact (so don’t have them doing weighted jump squats!)
Flexibility and balance work are almost as important, if not MORE important, considering most individuals with osteoporosis are of the age where falling and loss of balance is more common.