When are you visiting? Winter has short days and tends to be wet - so recommendations might change!
Other reasons for visiting Belfast apply. As well as being easy travel by train, if you are a railway/travel/transport fan this place is interesting:
https://www.nmni.com/our-museums/ulster-folk-and-transport-museum/Home.aspx
https://en.wikipedia.org/wiki/Ulster_Folk_and_Transport_Museum
If you have any Irish ancestry you can see how things used to be! The buildings allow a close up view of how things were were in many places, as they have been moved and reconstructed in the folk park.