You might have seen very similar pictures like the ones below as this little island where the Welsh equivalent of St. Valentine, St. Dwynwen, set up a convent after her tragic love live, is one of the most photographed ones up here in North Wales?
This is a lovely walk and you can make it as long (9miles) or as short (3 miles) as you like it.
For a short beach stroll out to Ynys Llanddwyn (only accessible at low tide) park your car at the Newborough Beach Car Park (charge at time of writing £5 for the day) – head down to the beach Traeth Llanddwyn (only partially dog-friendly in the summer months from May – Sept) and make your way along the shore to the peninsula you can see as soon as you're on the beach. To and fro are roughly 3 miles walk and you can take a picnic and eat it on the island on a nice day bird watching, maybe spot porpoises seals or dolphins or just enjoy the lovely views and scenery.
If you're in the mood for a serious walk, park your car at the Village Car Park in Newborough and make your way along the windy little lane, past the Roman Fort, to the beach through the dunes following the public footpath signs. Again, once you're at the beach you can just wander along the shore to Ynys Llanddwyn where you can spend time as long as the tides allow you to. You can then continue either along Traeth Penrhos where you can see the remains of 'The Greek Ship' if the tide is out dating back to 1852. and then head back inland through the forest after that excursion (this is the full 9 mile walk), just where the sand dunes end, there is a path on your right taking you through the sandy dunes towards the trees, or skip that part of the beach walk, and head towards the Newborough Forest straight when coming off the peninsula to wander back through lovely woods towards the Roman Fort and your car.
Want to know more about the legend surrounding this pretty part of the world? Read up on here: https://www.historic-uk.com/HistoryUK/HistoryofWales/St-Dwynwens-Day/