St Michael’s Mount is a tidal island 366 metres off the Mount’s Bay coast of Cornwall. The island is owned by the National Trust, however the St Aubyn family still live there. The island has a medieval church and castle and you can also visit the gardens. To get to the island you can walk across the causeway at low tide or take a boat (which is reasonably charged) when the tide is in.
The girls where with us when we visited, and they thoroughly enjoyed looking around. We would definitely recommend a visit.