tokens of love, ideas?

My boyfriend and I have been dating for seven months now. We are in university (same program, geology) and have known each other for a few years now. We both work in the field during the summer (camp life) and so we will not see each that much (probably a weekend or two, plus it is very hard to even have more than one short phone call a week in these super remote exploration camps). I want to give him a gift when he leaves for work, something that will remind him of me and make him feel closer to me when he misses me. Kind of like when women used to give men going to war a lock of hair or a piece of their jewelry to keep in their pocket while they were gone. Any ideas? I like to draw so I was thinking of maybe doing a portrait of him that was super small so that he could keep it in a book or something, but I have been struggling because I cant make it perfect enough and I would feel dumb if it wasn't just right. whatever it was it would have to be something very private. please help...