Best places to go in Northern Ireland?

Morning all!

Just doing some googling of Northern Ireland and wondering which places/attractions are must sees?? I'm into history, castles, nature and such. I know the region is HUGE on that stuff so I guess a little guidance would be helpful! I just saw pictures of the Giant's Causeway and nearly wet myself in excitement hahaha 
I'd like to do a little exploring (driving)... Would it be best to take the ferry and pay more OR fly over for cheap and rent a car? Any info is helpful! Thanks guys

The Antrim Glens (Giant's Causeway is in this area) are beautiful.  At least they were when I was there more than 10 years ago.  We drove up from Belfast and it was a lovely scenic drive.

My boyfriend is from Northern Ireland and his favourite area is Portstewart. I can't give advice on taking the ferry over flying and renting a car - simply depends on what you want. But you will definitely need a car to get a lot of different places over there!