I'm an adult social worker, and I work out of a rehab centre. Regaining and retaining mobility is my literal day job!
I recommend:
Plank for core stability - start on elbows and knees, or hands and knees and work up. Start with 10seconds, and work up.
Core stability is so important for so much, balance, pelvic floor, back strength etc. you don't need to start off with full plank for 3 minutes, just anything you can do. It will also help with sciatica.
If doing a plank is beyond you, then stand straight, tuck your bum in and tense your abdominals then purse your lips and blow out like you are blowing up a balloon. Ideally actually buy some balloons and use those- the force is greater and strengthens the core muscles more.
Reverse lunges to build leg strength and knee mobility - hold onto a chair if you need to, and again, do what you can and work up.
2 to 4 times daily get up off your knees or bottom without using your hands. If you can't do that yet, then do it off a chair. If you can't yet do that try using one hand, then a finger and build up.
Use stairs wherever possible and walk when possible. Get some weights you can walk with as well.
I also recommend working on hip flexors - Google a few exercises.
Increase your mobility, muscle mass and bone density if you can, and do your very best to retain it if you can't.