Howth is steeped in history and you can see many of its old ruins and buildings in just a couple of hours.
The harbour itself with its East and West piers, marina and quaint Howth Lighthouse is a good place to start to explore the village.
Then there's the 12th century Abbey and its graveyard in the heart of the village and the imposing 15th century Howth Castle in its beautiful grounds just outside the village.
The peninsula is also home to three of the many Martello Towers (c.1805) dotted around the Irish coast. Built by the British to watch out for the French invasion led by Napoleon he rather unsportingly failed to arrive! One is located in Howth village itself looking north, another is on the far side of the hill at Red Rock facing south while the third is located on Ireland's Eye. All are in excellent condition with the Howth tower housing a museum.
The Bailey Lighthouse (c.1802) is renowned the world over and is a spectacular example of its kind. Definitely worth a look.
If you like to walk or hike then Howth is the place for you. Walk in the footsteps of the legendary Fionn MacCumhaill and his band of heroes Na Fianna (Soldiers) whose base was Howth hill.
Take a trip to the charming beaches of Claremont and Burrow just outside the village for a dip in the sea or a a lazy day sunbathing.
A boat trip out to Ireland's Eye is also a great way to pass the afternoon and explore the island.
Beyond Howth
Outside of Howth and a quick journey into Dublin city centre itself and attractions abound.
There's the Book of Kells at Trinity College Dublin and Guinness' brewery at St. James' Gate.
Further afield the 5000 BC ! Newgrange settlement predates the pyramids by a couple of thousand years. Its located about 40 miles from Howth off the main Belfast road (M1) and is a real treasure for anyone with an interest in antiquities and early civilisation.
Going South the Dublin/Wicklow mountains are renowned for their beauty and exellent walking.
Why not take a day trip to Wales on the Irish Ferries fast craft (only 90 minutes crossing time) from Dublin Docks. The high speed ferry/catamaran leaves in the morning and returns in the evening. You can be back in Howth for the last pint.
Howth Castle 
Newgrange Brú na Bóinne c.5000BC