We spent 6mo in and out of the hospital last year. Oy.
Balloon for sure!!!
Nano-Bugs
Polar tech blanket (for wheelchair rides) ESP if a boy. For some reason THOUSANDS of girly ones get donated, but very few boyish ones (boys are in hospital, too!).. Pirates, skating, sporty... Dark blues, reds, black... Are all in VERY short supply. Polar tech washes and dries in jib time, which is important.
Mad Libs
Flavored Chapstick
Bubbles
Movies (often too tired to read)... Just check with hospital what kind of system they have
(( Most children's hospitals have Nintendo Wiis... Because they can be used one handed and only require MINIMAL wrist flicks. Ha. So the opposite of the intent of Wii. Hand me down / used wii games IF the hospital has Wiis are coveted))
Colorful flip flops
Pillow pets
Magnet games / travel games (full size doesn't sit on the bed, well) Boggle is a good example). NOTE: in children's hospital they usually have whole rooms full of boardgames that can be checked out. Regular hospitals it's 50/50.
Origami if he has use of both hands
ITunes gift card (phone games are SUPER popular, plus music, movies, etc.)
Cookie Bouquet
Glowsticks