This will probably sound odd, but always carry kleenex/TP. For many reasons. I won't elaborate.

Things I always carry on longer runs (in either my waist pack or Nathan vest): water, TP, lip balm with sunscreen, an extra gel, and a couple of bandaids/blister covers (never had to use them, but you never know). On really long runs I carry all that plus extra-extra food, a small pharmacy, my phone or a Spot (if I am in a remote place), a little tube of sunscreen, a map, a camera...you get the idea. I pretty much always wear gaiters.

For me it's about more than running. I approach each run as if it were an adventure of sorts. Sometimes they are really, really adventurous and other times the adventure might seem mild to most: seeing a really pretty chickadee or listening to the creek trickle.