Good point about historic race circuits. Traveling from Coleraine on the north coast, he can take in a section of the North West 200 circuit, where riders can achieve over 200mph on superbikes on a good day. On a road which is in daily use for 365 days of the year. Could also take in the Dundrod road circuit which is just north-west of Belfast if he fancied a trip into the countyside, scene of some classic motor racing back in the 50s & still in use as a bike circuit.
For castles, approaching Belfast on the coast road, is Carrickfergus, which has an 800 year old Norman castle, which is now a museum but until 40 or so years ago was still used as a military barracks. Belfast Lough used to be called Carrickfergus Lough, which gives you some idea of the importance this small town used to have.
Further down the country, the Mourne mountains are a nice change of scenery. Silent Valley and Spelga Dam make for a good drive.
Across the border, the Boyne valley has some neolithic tombs that are accessible (I think), never been but they look interesting.