Stockholm and beaches? Yes, it's pretty far north. Yes, it's large urban area. But don't forget, Stockholm is made up of islands. And you know what islands have? Beaches.
Longholmsbadet, in Sodermalm, might have more grass than sand, but use that to your advantage by taking refuge in the shade of its trees for a picnic.
Smedsuddsbadet in Ralamshov Park is a nice strip of golden sand in Kungsholmen. After a nice lunch at Cafe Kajak near Smedsuddsbadet, you can help the cafe live up to its name by renting a kayak and exploring the city by water. Sweden doesn't get too hot, but locals swarm the beaches in summer. If you do find yourself too hot for your suit, head to Agesta Nude Beach at the southern end of Lake Magelungen!!