I can see doing jelqs, SSJs, basic stretch straight up, out, and sides of those... Mandingo stretch, all easy to do laying back. Also [words=https://shop.mattersofsize.com/products/sizegenetics-penis-extender]Size Genetics[/words] I do laying down.
Myself I can't seem to do basic stretch straight down nor btc laying down. I stand at the foot of my bed and put one foot up on my bed frame or a table for those. Maybe you can?