Squats. Im too unflexible to have a proper form

Cardio.

anything high impact shreds my knees (running/treadmill) I have to go with ellipticals and rowers and whatnot

Anything resistance or weight with the legs.

Climbing steep hills or mountains.

The plank

Running. Or anything else with too much cardio. Or anything that puts a lot of strain on my knees, they've always been weak for some reason.

Squats. I have a weakass ass.

squats