Wall Sits are great for pre-exhausting the muscles before moving on to other lower body moves and it's also a great exercise for travelers because you don't need much space or any equipment. Obviously, if this move hurts your knees, you should skip it. If you're new to the exercise, only slide down a few inches until you get a feel for the exercise and know how your body will respond.
Do it right: Stand in front of a wall (about 2 feet in front of it) and lean against it. Slide down until your knees are at about 90-degree angles (or an angle that is comfortable for you) and hold, keeping the abs contracted, for 10-60 seconds. Come back to start and repeat.