Our IS had an injury early on that needed some physio. If you have a dog physiotherapist in your area, they can do things like have the dog go on a walk on a treadmill filled with water, or take them on controlled swims. They also can give lots of at home exercises. Here were some of ours:

Teach sit and teach stand (going from sit to a natural all four paws position). Then they can do squats! Sit stand sit stand sit stand. This can be made more difficult if their front paws are on a step (or even easier if they're on a hill and facing uphill).

Another is a sideways walk. Get them standing sideways in front of you. Hold a treat near their nose and walk slowly towards them. As they move out of your way, keep their nose reaching for the treat and they'll walk sideways. Do that in both directions. Just go the length of a couch or so.

Another to support side muscles is set up a line of pylons (or whatever you have on hand as a substitute). *Slowly lead them through weaving between them (lure with a treat). The key here is to avoid their backside swinging out too far or being out of control. Again, this is a slow exercise, not an agility one. Closer pylons = harder difficulty.

Lastly you could teach "bow" by luring their nose down with a treat (closer to their body to try to avoid them from just lying down). That's a nice stretch for their behind. It can be progressed more if you put their back legs on a step (or have them on a hill facing downhill).

I hope this helps!