Eat cherries. They're in season and you'll find them all over Hungary.
You can get a direct train to Budapest for a daytrip if you fancy – giving you an opportunity to visit the market designed by Gustav Eiffel (of Tower fame) and buy some cherries for the journey back. It's 3 hours or so each way, though.
There's plenty of little resort-type places on the outskirts of the lake selling lilos, dingies, buckets & spades, etc. Some of the beaches are part of resorts and you can't access them. You can probably find a nice enough place to eat just by having a wander. I went to a good Italian restaurant, but can't remember exactly where or what it was called. (I know, Italian in Hungary, but I'm vegetarian and Hungarian restaurants aren't particularly obliging in that respect).