There are several ways over the estuary to Fairbourne; the main line train, the boatman or on foot over the bridge. Once there, Fairbourne is the home of the narrow gauge Fairbourne Steam Railway. The walk on the footpath from Morfa Mawddach station to Fairbourne beach is as flat as a pancake, yet surrounded by views of the mountains. It is rideable on a mountain bike.
I have also shown as part of this page that you can walk quite a way into the sea on Barmouth beach! This is best done as the tide is going out.