Keeping a sugar solution in the fridge
I do exactly that.

I use a clear plastic squeeze bottles. I fill a mug 2/3 full with sugar, and top up with boiling water, stir, cool, funnel into the squeeze bottle, and keep in the fridge door. The squeeze bottle makes it easier to get the right amount of sugar into your beverage.



I don't even worry about keeping it sealed.  There's nothing much to oxidize. And the osmotic pressure of sugar solutions makes it very difficult for pathogens to live. This is why jams and jellies keep forever in your fridge. Like jams and jellies, the first thing you're probably going to see in the way of spoilage is mold.  In the case of simple syrup, we're talking dark thready stuff at the bottom of the bottle. This happens about a month out.  I chuck it, wash the bottle, and carry on.

On a related note, I have a blog post about iced coffee. And it's kind of a little wrong. Cold-brew coffee is pretty good for making iced coffee, but it's not better than leftover percolated coffee.  Seriously. We make hot coffee at my workplace using this cheapo percolator urn. We learned from the reviews on Amazon that we could just pour off the leftover coffee in the afternoon and have excellent iced coffee.  And that's exactly what we do. (To be fair, we use Gorilla coffee, so that might have something to do with it, but I think it'd be good with average coffee.)