Many guidebooks will tell you that one day in Belfast is enough time to see the city. They do this because they assume the average traveler will try to pack as much sightseeing into a trip as possible. The problem with this kind of travel is that you spend so much time going from sight to sight and city to city that you never get to experience the city. I think Belfast really needs three days which gives you two days to explore the city and one day for a tour to the Giants Causeway.
Belfast has really worked hard in the last fifteen years to change its image. At one time it was a city of with a history of violent conflict between the Catholics and the Protestants. Since the Good Friday Agreement was signed by the two sides in 1998 the city has been mostly peaceful. Both sides now share the power in the government.
The center city is very tourist friendly. There are maps on almost every corner that tell you where you are and what is in the area, something I wish more cities would install. There a lot of new and interesting developments including historic sites, walking tours, shopping and dining options.
The newest attraction in Belfast this year is the £77 million Titanic Museum. It opened in April 2012 and it is already attracting tourists that might not have considered a trip to Belfast. I really enjoyed this museum because it incorporated the history of the Belfast shipbuilding industry into the Titanic story. There are many other Titanic based tours and attractions to keep you busy for a couple of days.
Belfast is also a great place to use as a base to explore Northern Ireland. I did a full day bus tour outside of the city with the highlights of the trip being the Giants Causeway and the Antrim Coastal Road.
I would definitely recommend visiting Belfast if you are planning a trip to Ireland and do not listen to the guidebooks because you will need more than one day.