When I was in rehab I got a cigarette injector and would sell cigarettes. They regularly made runs to the smoke shop for us and took shopping lists. Yours may or may not but it was a good source of income and good for bartering.
Reading, drawing, playing card games were mostly how I passed my time. I'm assuming that since you're going to rehab you know how to play spades but if not I'd highly recommend learning.
They're of course going to give you a lot of coursework too so you can look forward to that.
Why used to? Hopefully they moved on to a better peeking place?