What are the best butt exercises that aren't hard on knees?

If possible move your workout into a pool. The workout will be less stressful on the joints, because of the water supporting your weight, at the same time the resistance of the water will make your muscles work harder.

Speed walking, or running helps.

Try some standing glute curls.

Stand next to something (like the back of a couch or chair) that’s about waist-high, that will support your weight.
Lean forward and support yourself by placing your hands on the support.
Take a small step backward while continuing to use the support.
Move one leg backward and then upward towards your back, until you feel your butt muscle tighten.
Do that about 10 times or until you feel those glutes get tired.
Repeat this using the other leg.

Climbing hills on bicycles. DO not sit down.

I have always had a firm bottom even when everything else is going to pot. If you tend to gain weight in the hips and thighs, you could have rock hard muscles, but if you have some extra weight on you, the fat will mask the muscle deinition, because the fat layer is over the muscle. I only mention it, because if you are tring to spot reduce, that does not work, there is not such thing. If anything you build the muscle ou concentrate on, make it bigger, and if you still have the fat, you make the area bigger.

I zumba, because I love to dance. I almost never do squats even though the zumba teachers seem obsessed with them. I do sometimes do them ballet style, with feet turned out, which in my opinion are much less stress on the knee. You go straight down, back straight (no stress on the back, improves posture, a regular squat you lean forward, it is competely unnatural to me). All the ballet dancers I know had rock hard defined asses, and they never did a foot forward squat. I don’t do any of the turns the zumba instructor does. I just stay straight forward when they turn.

Water aerobics and I also lap swim.

Walking.

We use our gluts every time we use our legs, in my opinion you don’t have to over focus on butt exercises if you move around a lot.

Ice skaters have decent lower bodies too.

“Step ups are another great exercise for the glutes. For step ups, place one foot on a step or platform and push through the heel to lift the body up. This is an excellent exercise for the glutes, providing you use a step that’s high enough, although you may need to work up to a higher step if you’re a beginner. You eventually want a height were your knee is at about a 90-degree angle.

The other key is to concentrate all your weight on the stepping leg. In other words, lower down gently, barely touching the toes of the other leg to the ground. You’ll really feel this when you take it slow and concentrate on the working leg.”

“While I’m fond of more compound moves (like the squats, lunges and step ups listed above) since they work more muscle groups, the hip extension is an exercise that specificially targets the largest muscle in the body…the gluteus maximus.

For this move, you can hold a dumbbell behind the knee or use ankle weights for added intensity.”

Source

Instead of bending on your knees and elbows on the floor and then doing leg lifts, you can try doing it on an exercise ball where your knees don’t touch the floor but you can still isolate your glutes with leg raises. When my knees bother me I do these and my knees have no pressure this way.

Another one would be to stand perfectly straight and tighten your glutes and hold for a few seconds and then release and repeat. If your knees aren’t that bad you can try doing this by standing on your toes and release on your way down. Tighten as you go up. You will engage more muscles that way and there is minimum pressure on you knees.