We have weekly Jamba Fridays where Jamba brings a bunch of the the 12oz size up to the school and sells them for $3. We get 20%, and it is zero effort and zero risk for us.
Our school has used Jamba Juice twice, during the end-of-school and start-of-school picnics at a nearby park. The kids pre-ordered their smoothies for the first one; during the second, Jamba just took a guess on how many they'd sell. (The start-of-school picnic includes the entire school and families, so it was a big crowd.) I wasn't in charge so I'm not sure, but I believe that each one raised $200 or more.
Obviously, tracking the money and the orders for the first picnic took a little work, but not much. The second one was completely effortless. Both nicely solved the problem of bringing drinks to the picnics. The JJ staff person was very nice both times.