Cambo Sands at Kingsbarns is a lovely weekend spot.
Crail is a nice stop as well you can add on to that, as well as the chocolate shop in Pittenween.
The East Neuk is further away than it looks on the map, at least it always feels like it. Tentsmuir south of Dundee is easier to get to and is a big beach with lots of forestry and dunes to wander in. Decent size seal colony and if your lucky you might just see the Sea Eagle.
An option if you’re trying to cram a beach in during a trip if you’re passing Edinburgh way is Silver Sands in Aberdour. There’s two beaches within an easy walk of each other and the Sands Cafe does some nice food. Not the best value for money admittedly.
Heading up the Angus coast also co sider Lunan Bay and St Cyrus (if you head here go for a look around Steptoes Yard, it’s err… definitely worth s look)