There are cross country ski machines that fold down and then fit under a bed.

With ellipticals it is important to do some exercises that fully extend the knee, because most of the ellipticals don't make you do that and so they don't work the inner quadriceps (vastus medialis). T
That muscle is used to extend the knee for the last 15 degrees and keeping it strong will help to support the medial collateral ligament (MCL) as well as the anterior cruciate ligament (ACL); which are the two most often injured.

Self resisted isometrics are good. Just cross one foot over the other at the ankle and using the one on the top to offer resistance to the one on the bottom, contract the one on the bottom in full knee extension. Hold for three seconds and relax for two, then repeat.
That method can also be used for active exercise. To do that just allow the knees to bend and straighten fully, as you resist the legs against each other.

With love, Erika