Going away gift for boyfriend?

I'm going away for a couple of months and i wanna give my bf a gift, a token to remember me by, but nothing too mushy or girly, since he's kind of macho. i want him to know that i want him to think of me as his, and when i come back that nothing will change about my feelings. what is a good gift idea? It can be symbolic too, not necessarily a gifty material item like a watch or something.