Collapsible reusable water pouches. I carry one, frozen, in my jersey pocket as a third bottle. By the time I need it the water has thawed and is usually still cool or even cold. And when it's empty it weighs less than a water bottle and is more compact.
If I want it for drinking only, I'll add electrolytes and it'll stay frozen longer. If I want plain water for drizzling on my head and torso I'll omit the electrolytes, but it won't stay frozen as long.
Very handy in Texas summer all day rides, on my road bikes with only two frame mount cages.
If I had to add cages I'd probably go for the handlebar mounts, although those can affect handling a bit. And if I'm doing a century I'll need room for a headlight and camera. Sometimes I can strap the Light & Motion headlight around the head tube to save space on the handlebar. The camera goes on the stem.