There are many gorgeous beaches in the area, the closest is Tywyn Beach approx. 15 minutes drive away (or a short walk from the Talylln Railway station). This is mainly sandy, although stonier than Aberdyfi (Aberdovey) and is covered when the tide is in. There are bigger waves (making it popular with surfers), rocks to climb and the possibility of seeing dolphins out at sea! There’s also the really really old remains of a forest (6,000 years old) that can be seen at low tide. This was only discovered in Winter 2014 when the storms washed away a layer of Peat and revealed it underneath.
Aberdyfi beach is only about 5 minutes further south from Tywyn and accessible regardless of the tide and is sandy, with a playground, donkey rides and a lifeboat station. This is in the estuary and looks across to Borth beach. Parking can be tricky here on busy days so the mainline train might be worth considering.
Fairbourne beach (30 minutes drive) is sandy at low tide, but there are only stones at high tide.
Barmouth beach (45 minutes drive) has great sand dunes to enjoy.
Harlech Beach (1 hours drive) is large and sandy and a great idea to combine with a trip to the castle!