If I had to choose just one excercise, it would be walking on hilly terrain. I’ll use Prospect Park in Bklyn as an example, cause that’s the one I’m most familiar with.
Start with the loop (its 3 miles around). Once you’re acclimated to that, graduate to the inner paths, stairways and grass fields. The up / down and uneven terrain will give you a good core workout, which in turn encorages good posture.
If you can do the loop in under an hour, you’re A-OK. And belive me, 3 miles on hills is NOT the same as 3 miles on flat surface or a treadmill.