When I can't sleep, I like to do these simple and relaxing stretches:
1.Lay flat on your stomach and place both hands next to your shoulders. Slowly push your torso up, and lift up your chin while exhaling all the air out of your lungs using your diaphragm. Breathe in and let your head hang downwards, and then breathe in again. Be careful not to overstretch your neck! Slowly turn your head to look at your ankles, turning your shoulders slightly. Do both sides a few times.
2. A good follow-up to this stretch: Start out laying flat on your stomach again. Bend your legs towards your back and grab your ankles. Slowly pull your arms back using your legs.
3. This might sound really weird, but flexing your butt muscles feels awesome! Lay flat on your back and squeeze them together as tightly as you can for 10 seconds. I know what you are thinking, but seriously just try it. Somehow a lot of tension builds up in there hahaha
4. One more good one, it might be difficult, but it's great for upper back and neck tension: Starting out laying on your back, bend your legs and bring both knees to your chest. Hug your legs for a few seconds to get accustomed to that position. Using your hands right behind your hips, lift up your hips so that your knees are touching your face. Bring your knees as far up as you can, let them rest on your forehead. You can use the wall behind your head for support.
other stuff: Don't use your bed for mentally active or work-related things; reading being the exception because sometimes it helps you fall asleep.