Tour the Guinness brewery and enjoy a pint in their rooftop bar with a beautiful view of Dublin.
If you're in town early you can visit any tourism center in Dublin to arrange a day tour to the Cliffs of Moher, which will cost anywhere between €15 to €40. A must-see.
There are free walking tours available most days.
Paddy Wagon offers day trips by bus to various tourist attractions from Dublin.