Wow, I didn't know some campuses wouldn't allow it. My SAI chapter has sold soup, grilled cheese sandwiches, coffee & tea, you name it, and have always made money. And no one ever got sick.
Although I can see how this could be a liability issue.
Anyways, BabyP, I recommend 1)serving something that would sell well, and 2)pricing it cheap. I think the smoothie idea is really cool, just make sure to keep the perishible parts of it well refrigerated during the sale.
Another fun idea: on your campus, have a Kool-Aid and cookies stand! The Phi Mu Alpha chapter here did that, and they made money from it!! They priced Koolaid at like 50 cents, and cookies were like a quarter a piece or something.
My SAI chapter sells food type stuff a lot, so PM me if you want more suggestions!