First, the "outdoor activities" seem to be a big part of the problem right now. People hanging out at the beach, the playgrounds, the parks, eating in groups in outdoor seating at restaurants, protesting, etc. all accelerate the spread.
It may well turn out that people doing fewer of those things means that we don't have a heavy second wave in the fall. I hope that's the case.
Second, the window isn't "closing," it's pretty much slammed shut. The genie is out of the bottle for the U.S. and likely the planet. We're in the ballpark of 50k diagnosed cases per day, plus between twice and ten times that number of undiagnosed cases, if not more.
Contact tracing 50k cases a day would be a massive undertaking, and we'd still be doing nothing about the 50k+ undiagnosed cases out there, most of which represent people who are still spreading the virus.
The political will for additional lockdowns and preventative measures is all but spent.
I hate to say it, but absent a very effective vaccine, COVID-19 is most likely here to stay.