I have small vials with eczema medication (essentially a weak version and a strong version).

People like /u/messiahcakes and /u/aborca have mentioned caffeine pills. This is probably not the place for discussion, but it's insane to me the various drug laws, particularly in the US. I carry Modafinil with me, which I bought from India (where the patent expired) and shipped to UK where it is not illegal to use or buy.

It is much better than caffeine. It would take me about ten cups of coffee to achieve the same effect as half a pill of Modafinil, and from using it over the course of several years, I will say that it is not addictive and has very few side effects (you can confirm this with the research).

I'm not saying this because I want people to go out and get illegal drugs (it is illegal to buy without a prescription in the US) but Modafinil has saved me on numerous occasions when I have needed to travel while tired.