The Giant's Causeway — 40,000 hexagonal basalt columns on the north Antrim coast, formed by volcanic activity 60 million years ago — is the obvious day trip from Belfast, about 90 minutes by car or bus. The Causeway Coast road along the north coast, taking in Carrick-a-Rede rope bridge, Dunluce Castle, and the Bushmills whiskey distillery, is one of the most spectacular coastal drives in Europe.
The Mountains of Mourne, about 60 kilometres south of Belfast, offer good walking. The Mourne Wall — a 35-kilometre dry-stone wall built in the early 20th century to enclose the water catchment — is the centrepiece of a fine hill walk. You can also visit the Dark Hedges (an avenue of 300-year-old beech trees used as a filming location in Game of Thrones), which is about an hour's drive north.