Yes, that would have been Lake Balaton, as /r/jimmybabs mentioned.

GDR citizens needed an 'exit' visa in order to be able to go to Hungary. This exit visa was not that difficult to obtain (being know as a 'socialist' foreign country).

What the problem with Hungary was for GDR citizens is that it was very expensive. The exchange rate for the GDR mark was only 700 forint, while for the FRG mark it was much higher. In the GDR the exchange rate was 1-1 for both currencies.

Thus, when GDR families went to Hungary, they brought as much as they could with them since Hungary was quite expensive for them.

At Lake Balaton Germans from both countries could mingle and meet each other and many made 'holiday' friends, when they would see each other every year.