Frist voices here that think lifting the travel restrictions was a mistake. While we had around 100-150 new cases per day a month a go the number is up to over 500 yesterday. Also in headlines a pupil visiting grandparents in Serbia got infected there and after returning to Germany infected many more attending a birthday party. Neighbor was two weeks in Greece on Crete and he said there were nearly no restrictions in place and the locals doesn't seem to care - he said he and his family were only visiting the beach and eating in their room for fear of infection - only in the airport theere were some safety measures.
I still hope visiting some German parks in September but its more than likely that by that time might have the fallout from the summer tourists and having a new full lockdown by then. If that happens I assume international travel will be restricted until there is a vaccine - latest predictions for that are mid-late 2021.
Just over 500 cases a day for a population of 83 million is not a lot at all. In the UK we have a higher 7 day average than Germany and our population is smaller. And when you look at countries like Spain, Netherlands, France, Belgium, they all have higher 7 day averages when scaled to population than Germany. The only EU country actually making better progress against the virus is Italy, so I really wouldn't panic so much if I were you.
If cases go above 1000 a day then there is a cause for concern.
I think you should be ok for September, Germany has a robust testing and tracing system and a strong mask policy, something a lot of other countries don't have, which will keep infection rates low. I would only worry about a second lockdown in the winter when we also have the flu going around, that's when the healthcare system could be overwhelmed.
There's also a couple of vaccines, Oxford in particular that I know of, which are looking promising for end of this year. Not worth being too optimistic or pessimistic about them but if successful early next year would be possible for vaccine distribution. It is completely possible we will all have to sit through another lockdown, but chances are it won't be until October-november time, so I think you would be ok for parks in September.
I'm also hoping to do some parks in September when the rush has died down and people have gone back to school etc, hopefully they will be a lot quieter than they are at the moment.