It's the anti swamp-butt of beverages
Summer is gross. Yes, yes, there's fun in the sun and beaches and pools and lakes and various other water-adjacent activities, but that's just a fraction of it. Most of the time you're hunkered indoors around an air conditioner vent (or worse, an almost comically inefficient ceiling fan) wearing the social minimum of fabric and wondering if your lower back and gluteal region is ever going to be dry again. Y'all coffee fetishists can have fun trudging out into the hellswamp for your fancy-ass cold brew concoctions. The rest of us will be inside chilling and sucking down our dreggy coffee fizzes.
A few years back, coffee heads everywhere went nutso for coffee tonics, which are a meld of cold brew coffee and, shockingly enough, tonic. They're great! All bubbly, tangy, and chill. What I'm talking about is way lazier, nearly as delightful, and takes little to no forethought—just a small amount of willpower to leave the last cup or two of coffee in the pot from the morning. (Pro dirtbag tip: Pour leftover cold coffee into a lidded jar or pitcher over the course of a couple days and keep it in the fridge, or even better, freeze it. This is not the time for preciousness.)
Ready? Put some ice in a tall glass, and if that is crushed or pebble ice, that's even better but by no means imperative. Pour the leftover coffee from the morning in there, about halfway to two-thirds of the way up, then top that with seltzer an inch or so short of the top, because it fizzes up in an unexpected and delightful way. Squeeze in some lemon if you happen to have one on hand, and sip away like the sweat-free, cooly-caffeinated genius that you are. And sure, if you want to put your favorite dairy or nutmilk (a.k.a. milk made of lies but whatever gets you through the day) in there, it is your prerogative as a free American human, but leave out the lemon because curdled milk coffee fizz is really gross.