At low tide, a narrow strip of sand connects Castle Beach to St Catherine's Island, a small, rocky tidal island topped with a Victorian fort that was built to defend against a French invasion that never came. The walk across is short and the views back to Tenby's harbour and castle are as good as anywhere in Wales.
Check the tide times before you go, it's only accessible for a few hours either side of low water. The island itself isn't large but the experience of standing on it, looking back at the town, is properly memorable.