What is a good exercise for arthritic knees?

Leg raise (lying)
leg raise
Lie flat on your back on the floor or bed with your arms at your sides, toes up.
Keep your leg straight while tightening your leg muscles, and slowly lift it several inches.
Tighten your stomach muscles to push your lower back down.
Hold and count to 5, then lower your leg as slowly as possible.
Repeat, then switch to the other leg.

Hamstring stretch (lying)
hamstring stretch
Lie on the floor or bed with both legs bent.
Slowly lift one leg, still bent, and bring your knee back toward your chest.
Link your hands behind your thigh, not your knee, and straighten your leg.
Pull your straight leg back toward your head until you feel the stretch.
Hold for 30 to 60 seconds, then slowly bend your knee and lower your leg back to the floor.

Stand with your feet shoulder-distance apart and stretch your arms out in front of you.
Slowly bend your knees until you’re in a half-sitting position. Hold on to a chair for balance, if necessary.
Keep your back straight and chest lifted — don’t lean forward.
With your feet flat on the floor, hold the position for 5 seconds, then slowly stand back up.