One thing to keep with you would be swim goggles in regular or prescription versions. For those of us who wear glasses, you can get goggles in standard diopters to closely approximate your Rx.

As most Rx goggles come in .5 diopter increments, if you have eyes that differ by more than 1.0 or so, you can get two pairs of different diopters and swap lenses to make two more exact pairs, each with a much closer Rx to your individual eyes instead of just one that's a comprimise between the two diopters. To tell each lens apart you can subtly mark one as R or L or do what I do and have the logo on the nosepiece right side up in the correct lens orientation.

In Rx or non, these can serve as emergency specs if your glasses were to break as well as aiding navigation through smoke and other such eye irritants.

Plus, I've taken them on numerous backcountry trips and always found a reason to check out bodies of water using them.

These are only a few possible suggestions for a useful, light and only slightly bulky item. Anyone else carry a pair or have other ideas for usage?