Miso is a decent choice but if you like it and its available , I have heard clear soup is a better alternative. My girlfriends and I always do our dinner nights out at japanese because of the healthier options. I always order the seaweed salad and clear soup, and a tuna roll and salmon roll , both with brown rice. Its ok to ask for the brown rice they should be able to accommodate you, its just as good if not better tasting!
I would recommend leaning towards simple and staying away from any of the fancier rolls that include some kind of sauce. Most sauces involve the japanese mayo. Seared tuna or tuna shashimi is also delicious and good for you.
Regardless - don't forget to enjoy your dinner out and don't stress too much over it