I know not everyone looks at the Belfast forum so I just wanted to highlight that you need to pre-book tickets for the Titanic Experience in Belfast. I don't think is maybe widely known and have heard complaints from people who have made day trips from Scotland and the south specifically to see the exhibition. I would expect that the novelty factor will wear off but with this year being the centenary, you are best to prebook - especially during weekends and school holidays.
But remember there are lots of other Titanic-based attractions including the excellent Titanica exhibition at the Ulster Folk and Transport Museum in Cultra (15 minutes by train from Belfast), Titanic boat tours, the Titanic Walking Tour etc.
The newly opened Titanic Memorial Garden at the city hall is also worth a look - nice and understated memorial naming all those who perished.