Merlin it has loads to see and do, definitely enough to fill a weekend.
If you have children with you, there is a fantastic museum about a mile and half from the city centre, in the university area, the Ulster Museum, and best of all it is free. And it is also next to a lovely park, Botanic Gardens, for a wee walk.
Titanic is a fantastic visitor attraction, it's not just about the ship itself, although obviously that is the focus, but has loads of general history of industry in Belfast. There are loads of interactive exhibits in it that are interesting enough to entertain children.
If you fancy a wander round the shops, there are plenty of those, both the usual high street stuff and plenty of little independent ones too. Really good restaurants all over the place, in all sorts of price ranges. If you are in the city on a Saturday morning, there is a really great indoor market, St Georges, which has got food, craft stalls, artists etc, and usually has live music too.
There are some really lovely bars in the city centre, from the traditional ones with the Guinness to the upmarket ones with the £10 gin and tonics.
And there are plenty of hotels in the city centre, some expensive, some budget, within walking distance of where you might want to go in the evening.
I could probably write about 2000 words here, but I know I have already hijacked the thread! But if you want to PM me I'd be happy to give you more information 