An underground tour in Csodabogyós-barlang can save a rainy day, but it can save you from sunstroke during a heatwave, too. Hopping in from the beach is an option, but keep in mind that during the 90 - 120 minutes long tour you'll have to crawl, climb and slither about so much that it'll feel like a hardcore Pilates class.
The cave close to Balatonederics is on the eastern edge of Keszthelyi-hegység, and it is the 10th longest (5200 m long, 121 m deep) cave in Hungary. It was named after the thorny bush with red wonderberries in front of the cave's entrance. You don't need to be a geology maniac to enjoy Csodabogyós, because apart from the "magnificent stones" and the "formations from the Triassic Period", you'll experience a fun physical test with lots of climbing and bending.