Susanne is almost correct, but misses a few details. "Can" does not relate to how many options there are.
"Could" is about intentions or possibilities - We could go to Tokyo, but only if we want to. We could also go somewhere else.
"Can" is about ability - We can go to Tokyo, it is something that we are able to do.
Your confusion might come from the fact that "could" is also the past tense form of "can". We could go to Tokyo last year - We had the ability to go to Tokyo last year.