Titanic museum - pay for the headset, really worth it(and you only hear ‘that song’ once!
Red bus tours are good.
Hotel Europa (once the most bombed in Europe), really central and next to the train and bus station (good connections to the airports and north coast - but check timetables!).
North coast is spectacular- giants causeway is good - you don’t have to go in the visitor centre so can visit for free.
You can travel from Ballycastle to Rathlin island to see the seals (and puffins earlier in the year). Morton’s in ballycastle is a great fish and cop shop.
Beautiful beaches.
Port Balintrea to see the iron isles.
Some amazing beaches and fab restaurants- often have £10 deals for 2 courses which are scrum. Ramors in Port Rush is very popular (it maybe portstewart, I get the two mixed up even though we visit 2-3 times a year!). Oh you pay extra for sides (like in America) and champ is mash with spring onions (aka scallions).
Always pop into some tea shops off the beaten track - they are generally fantastic!
Oh and don’t tell anyone how lovely it is - don’t want everyone going!!!