the antrim coast road is a must - from Larne along to Portrush - one of the best drives in the world imo (and I think it was voted as such somewhere recently),
http://news.bbc.co.uk/1/hi/northern_ireland/6141692.stm
Will only take a few hours, it also combines the giants causeway and bushmills if you wish to take a look.
If you're interested in golf there's as many golf courses as you'd want in the portrush/portstewart/castlerock area too.
The Inishowen peninsula is also pretty spectacular, as is most of the donegal coast.
Derry city is also pretty good for nightlife etc, but to my shame I know nothing about the coast from belfast down to carlingford.