Lihou Island in Guernsey is an interesting one. Accessible by foot only during low tide it lies in front of Saint Peter. It's a beautiful little isle with blooming meadows, areas protected for birding, old ruins and a self-catering house for accommodation.
The crossing in itself is an adventure and one can spot anemones and other little treasures in low tide pools on the way. Good shoes or wellies are required as there's either some waddling through water or jumping from stone to stone to keep your feet dry. Bring your lunch here and enjoy it on top of the hill while watching the birds and boats go by. The ultimate relaxing experience and highly recommended!