It used to be so painful to climb just ONE flight of stairs, now, I do several. On purpose. What?!?!
My knees were what bothered me before, but now it's what I've been doing to get rid of the stiffness from sitting so long at work. After 2-3 hours of sitting, I have to get up or else my feet go to sleep and my joints start aching.
So, I use the stairs. Up a couple of flights, walk a hallway, up a couple more flights, walk a hallway, down a couple flights... you get the idea. I do this for about 15 minutes at least a couple times each day at work.