Healthy human bodies are designed to move, and being intolerant to exertion when you have CFS/SEID is a major catch 22.
Still, there may be a few things you can do to keep your muscles and joints from becoming completely deconditioned and seizing up over time.
I drew some pictures of some floor based exercises and stretches that I do when I feel well enough. It's much easier for me on the floor, as like many other people with this condition, I have developed orthostatic intolerance, meaning my body does not cope well with being upright.
These exercises pictured are gleaned from having attended yoga and pilates classes over the years. Please be advised that I have no formal training in either of these disciplines. If you exprience any pain, please scale back the movement or stop. However even just 1-2cm of gentle movement, or doing the movement in your imagination can be beneficial. If you have CFS/SEID, you must go slow, be gentle, have lots of rests between the exercises. If you experience any symptoms (for example, my neck and armpit glands tighten and become sore when I've done too much), or post exertional malaise the next day, SCALE BACK. It can be useful to monitor your heart rate, to make sure you aren't going to hard. DO NOT push yourself. Also, engage your pelvic floor whilst doing the exercises to protect your back and build core strength. (imagine holding in a fart). More ideas can be found by looking for recumbent exercises for POTS (Postural Orthostatic Tachycardia Syndrome) on youtube. Its good to try gently using all your joints, at least once a day.
I also intersperse the exercises with restorative yoga poses, or just do restorative yoga if I don't feel well enough for anything else. A bolster is handy.