Tomogashima is a group of islands off Kada, reached by ferry when the weather allows. The draw is not beaches but the mix of forest paths, brick gun batteries, tunnels, and sea views left from the islands' role in defending Osaka Bay.
It takes more effort than the city-centre sights, so save it for a clear day and start early. The reward is Wakayama's most unusual excursion, with a little ruined-fort atmosphere and a lot of open water.