Many people like the idea of soda fountains, but the official fountains that mix soda and pre-mix concentrate are quite expensive and work to maintain.
To gain some of the convenience and efficiency, how about a quasi-real fountain using existing 2-liter pop bottles? Such a fountain would be designed to look like a regular one. Inside, ordinary 2-liter bottles would be attached. In a "party" model, they would just be in an icebox, in a full-time model it would have refrigeration.
To attach the bottles, their caps would be removed and a special cap screwed on that would have two hoses -- one for pop to come out, and the other for gas (CO2 or air) to come in to push out the drink. A CO2 cylinder could power this, or an air pump if you want battery power. Gravity might be able to do it but pressure certainly could.
The special caps would have another quick connect interface that lets them be turned upside down and snapped into place. Alternately, the bottles could be in another orientation as long as the feed tube can reach their bottom.
You might want to be able to have two bottles of the same drink feed the same fountain to give more capacity and the ability to drain one and then the other so that they can be "hot swapped" (really cold but you know what I mean.) Or people might prefer to be able to offer lots of different drinks, but just 2L of each.
The goal of this design is to be very cheap. The one based on ice and CO2 cartridges would be very cheap and need no power -- more of a party amusement, but also an efficient way to give people only what they need.
Of course you could just empty the bottles into reservoirs, but then you need a way to clean them. Cleaning the tubes could be done with an attachment for a garden hose to flush them, and the caps could go in a dishwasher.
This is my lowest tech idea yet...