-A cigarette lighter and a book of matches as backup. Both are light, and you can use them to tart a fire if you are camping somewhere.
-An empty tin can. Reason being, you can punch holes in it and make a stove out of it. See the link below:
-A hand-operated can opener.
-Socks were mentioned, so I'll say an extra t-shirt and an extra pair of underwear
-Weed. You can carry a lot of high for not a lot of weight/space.
-A condom, just in case the opportunity to use it presents itself.
-If you have pale skin, sunblock
-Hand sanitizer
Definitely agreed. Especially if you are hitchhiking, bike touring, or take the bus. In the case of the latter, I got caught in the bathroom on the back of a Greyhound having used the toilet, with nothing to wipe with. I had to crack the door open and ask strangers to hand me something to use. One of the cute 20-something gals sitting in the back handed me a roll of paper towels while I awkwardly had the door cracked open. 'Twas a long walk back to the front of the bus where I was sitting, because everyone knew... A sealed wet wipe in my pocket would have prevented all of that.
You can also use them to give yourself a cleaning if no shower is accessible and you can't find a suitable public restroom with a sink to "bird bath" with.