I guess you could put something else other whiskey in a pocket flask. Maybe some nice sipping tequila in summertime, or perhaps a bit of brandy around the holidays. Whatever you fancy.
But whatever goes inside, our vote is: you should own certainly own one. Not because you should be sneaking spirits in places you shouldn't have them (although...) But because its summertime, and you should be headed out on adventures. And once you've achieved whatever goal you set, it's good to celebrate a little.