OK ... coming from a fairly rusty background (I have a 1967 morris minor which was originally more rust than metal), a good way of dissolving rust is phosphoric acid ... there is a bunch of phosphoric acid in cola.
I would remove the chain (if you move along the length of the chain you will find one link that is more like a clip than a link, pop this one apart and the chain will come off without you needing to take the bike apart), try flexing it a bit to loosen up the rust and then leave it to soak in acid for a few days.
Ideally you might to bend the links of the chain every day (wrap it round a pipe) to loosen stuff off.
Once you are done dissolving the rust, you need to wash that sugary stuff off the chain & let it dry. Then get a pot of oil .... used engine oil is ideal ... dump the chain in there for a week or two, again it would be a good idea to move the links about.
After a while you should have a chain that's fine & dandy, replacing it is just the disposable culture talking, with just a little work you can save both the environment and your pocket!
Next year, just take the chain off when you get home, wash it in vinegar to neutralise the dust and then dump it in a pot of oil (jam jars are ideal for this) & tape the pot to your bike so you dont loose it