Leg exercises with bad knees

CeeCeeMee

I'm looking for some help with ideas of exercises to help tone my legs (thighs especially) without doing more damage to my 50-year old knees.

My knees aren't as bad as they could be, but I still have problem with so many lunges, lifts, etc., so it limits many of the choices others can take in stride.

Anyone have some suggestions? Thanks in advance!