Not sure if you get it in the US the same as in the UK, but I find that even just squash is good with carbonated water, e.g. orange squash. Cheaper than syrups and it tastes good with still water too!

I generally follow this recipe, but normally do about double the amount of oats for the quantity of honey and oil, as I’m looking for it to be a bit less sweet and a bit drier. You can add anything after it’s cooked, and before it’s cooled - I normally add loads of sultanas and dried cranberries, and a dried tropical fruit mix. I also sometimes mix in some desiccated coconut.