[He still hasn't seen the ocean, and he knows Marco would want to go too, so it might be a betrayal (brotrayal) to take a girl there (not just any girl, Mikasa) just to impress her. But the idea is tempting enough to linger in the back of his mind, spawning self-indulgent fantasies of how awed she would be.]
[Written]
You think sandwiches will be good enough?