So, if you're just joining them for dinner, you're NOT expected to bring a plate.
However, even for a situation like that it's often expected to bring a wrapped gift for your hosts, so certainly if couchsurfing I'd consider bringing something.
Ideally, gifts that can't be purchased in Japan would be a nice idea, and it'd be very special if from your home country (or city even!). If you know there are several people or a family, a gift for each one is even better.
In terms of price, err on the inexpensive side - you don't want them to feel that they owe you something as a result.
If you haven't got something from your country and just want to pick something up in Japan, consider fruit or food, cakes, or candy as easy but acceptable gifts.
*Things to avoid:
This will vary to some degree, but ask around and you'll be told - don't bring lillies, blossoms or lotus plants - sometimes associated with funerals. Potted plants are associated with sickness taking root. The numbers 4 and 9 are to be avoided (eg number of flowers).
Always offer the gift with two hands, and if you receive one in return, it's common to protest a couple of times before graciously accepting.
Source: The internet, and three years of Japanese language and culture study.