Two miles west of Holyhead, the sea vents its fury against the vertiginous South Stack Cliffs, an important Royal Society for the Protection of Birds (RSPB) reserve where up to 9000 seabirds nest. Between May and June guillemots, razorbills and 15 loved-up puffin couples congregate here, while choughs, fulmars, peregrine falcons and numerous other species may be spotted throughout the year. You can get information, hire binoculars, and pre-book guided walks at the visitor centre.
A steep, serpentine flight of steps leads down to the suspension bridge that crosses to the South Stack Lighthouse for tremendous cliff views. Numerous paths lead into the bracken-covered, hilly interior, climbing 219m Holyhead Mountain (Mynydd Twr) and skirting Neolithic stone circles. For a sheltered view, head to Twr Ellin (Ellin's Tower; April to September), a birdwatcher magnet equipped with binoculars, telescopes and a TV with a live feed from cameras on the cliffs.
Caer y Twr, one of Wales' most impressive Iron Age sites (open 10am to 4pm daily, admission free), is found at the blustery summit of Holyhead Mountain, but if you're not up for a 45-minute hike, there are also remains of Neolithic round houses a short stroll along a path that's opposite the car park.