I would get one to share between my family for the parks and all of them would have them at the resorts. Although we would probably just fill it with ice water in the park most of the time anyway.
The cups would need to be tightly sealed with a cabineer (spelling) clip, small in size and weight(maybe 20oz), heavily insulated, and be good at all parks and resorts.
A set fee based on your length of stay or pay per day packages would be needed. A dining package option upgrade would be nice as well.