The Dirty Onion is pretty cool and in the oldest building in Belfast, but its more into a hipster craft cocktails place. My favorite pub was probably the Duke of York. The alleyway has some cool murals and such, very picturesque. I also enjoyed a place called Kelly's Cellars. Also look into the murals on Shankill and Falls Roads to learn more about the conflict there in Northern Ireland from both sides. It is fascinating stuff.
DEFINITELY go to the Titanic museum. It was amazing. You can spend several hours both inside and out. It was the highlight of my trip to Belfast.
Check into a day trip to Giant's Causeway. It is only a couple of hours away but has some pretty stunning scenery. EDIT, just saw that's already on your radar.
Overall, Belfast is a pretty dreary city with a few fun things to do. I spent two days there and felt like I had just about done it all. Have fun walking around, enjoy a pint or two and report back!