Bangor Marina is the reason people come. Walk the pontoons past hundreds of boats, find a café on the waterfront, watch the light change on the Lough. On a clear day you can see across to Scotland, which is closer than most people expect. In summer the marina has a quality that surprises everyone the first time: a Mediterranean looseness that the Northern Irish weather only occasionally permits but makes the most of when it does.
The cafes along the marina edge are the places to settle in. The view is better than any interior. Go in the afternoon when the light is on the water and the boats are back from wherever they've been. It's also the start and end point for the North Down Coastal Path west to Holywood, so you can earn your coffee first if you want to.