Belfast is a marvelous small town, and it is "real."
The downtown area is small. Just park where you can and wander the couple of main streets. There are bookstores and antique shops, an interesting "green" store, artists galore, and many other shops, including a tiny cheese store. Be sure to notice the artistic benches placed at the main intersection in town. People do sit on them.
There are more than enough restaurants to take care of every meal, depending on your whim. Seafood at Weathervane right on the water or Dockside, a little up the hill but still waterview, Italian at DaVino's, American at Darby's, vegetarian at Chase's, even Thai and Chinese, all within a few blocks. I haven't had a bad meal at any of them.
In the evening, if you don't spend most of it enjoying a meal at a restaurant, there is an actual old downtown movie theater, The Colonial (www.colonial.com) which plays mainstream and sometimes more indie types of movies. You can also just hang out and look at the boats, including working fishermen.
There are short boat trips available. You can get brochures almost anywhere in town or right down at the dock. You can also walk across the fairly long footbridge that you'll find under Route 3.
Have fun. I love Belfast and was lucky enough to stay there awhile last year.