I often carry around a couple of reusable hand warmers when hiking or just around the outdoors when it's cold. They don't take up much space, they're easy to reuse (just boil) and they provide a fair bit of heat for a while.
However, try as I might I can't find any equivalent reusable packs that work the other way - i.e. they set of a reaction that draws in heat and makes them cold for a while, and then (I'd expect) you could put them in the freezer for a while to reverse the process.
I wouldn't really use them for hands, but they could come in useful for cooling you down generally - placing at the back of your neck for instance.
Do such things exist? Technically I don't see why it's not possible but I haven't seen any around at all.
EDIT: After a bit of looking around I stumbled across these - perhaps they do exist after all! Has anyone used them? It looks like they're geared more towards medicine than general use but if the effect's the same that's fine by me.