Learning something new of a physical nature (new sport or physical game). That helps. I can see my mind wander and drift quickly.
You just have to let go of mind concentration and you'll drift off. Now if I can figure out how to FORCE the drift then I'll be good.
When I can't sleep - my mind is stuck on one thing or another. Like the brain isn't lubricated.
Sometimes low level pain just perceptible (normally not enough to warrant a pill) enough to keep you awake.
Delayed onset muscle soreness can keep me up as well - but not exercising can keep me up too.