The most common form of seaweed I use in nori (or dry seaweed). This can be used as topping for many Japanese dishes, such as soba, okonomiyaki and ramen. It adds a delicious flavor to the dishes as well as nutritional content.
Another form of seaweed I love is hijiki, which is a type of seaweed that is boiled and dried. This is used in side dishes (mix hijiki with carrots, abura age, beans, etc.) and it will add so much flavor and nutrition to your meal. Furthermore, hijiki can be mixed in rice to completely change the taste of the rice. Often, Japanese restaurants have hijiki as a side dish, so hopefully you will be able to taste it during your stay in Japan!