V.W.
To address only one part of your questions (what can students do with the money that doesn't involve buying stuff first?)...
I have a 9th grader, so he was 8th grade last year. :)
The most valuable thing to my child is time. Really. He wants to be in charge of his own time. Really, who wouldn't? Isn't that WHY we work? So we can have leisure time? Well... I could go off on a Lutheran bent about vocation and working to serve your neighbor... but our selfish natures really work because we don't want to work. Get it? We work to stock up on leisure time.
So... can you let them buy leisure time? Extra lunch? Early release? 3 minutes gratis at the beginning of class? Free pass on a homework assignment of their choice (buys time at home)? I am not sure how that would work within the bounds of the rest of their curriculum, but TIME is what would appeal to my child. Autonomy regarding the use of time.