Other options are iced tea with no sweetener. You can get different types of ice teas as well to slightly change the flavors up.
If you don't like hot beverages, make it iced. If you don't want to make some all the time, use a bigger kettle.
If you want to reduce ongoing costs, could piece together a feeder system for a larger co2 tank, or a refill tank with kits to refill their bottles or adapt other bottles for the soda stream.
slices of ginger
apple cider vinegar
cucumber slices
Does anyone really 'dislike' water? It doesn't taste of anything.
Also: Water isn't just water, there's quite some difference in taste depending on the water source. To this day there's certain "brands" I like drinking and others that I'm having a hard time drinking because I don't like the taste.