Been there a lot. I'm a North shore person so most of my frame of reference comes from there.
Thumbs up on September. Usually, the crowds thin after the Labor Day weekend, and the lake becomes much more peaceful and everything becomes less crowded. Best time of year in my opinion.
You may be a bit early, but one of the cooler things to see is the fall Kokonee salmon run in Taylor Creek, just south of emerald bay. Pretty cool.
check this out:
https://jamesscho.wordpress.com/2015...r-creek-tahoe/
At the northwest side, you have Tahoe City and just a few miles away you have Squaw Valley. Lots of things to do. A river rafting trip down the Truckee starting from the Tahoe City dam is low key and fun with small kids. They may be packed up by then with the rafts, but if not, check it out.
At north lake you have casinos. A little downscale, but still okay. They're at stateline or in Incline Village. One of the nicer beaches on the lake is the Incline Village beach.
20 minutes from Tahoe City or Northlake is the old town of Truckee. It's become a pretty nice town for food and strolling. One of the best restaurants around the lake is the Cottonwood...it overlooks Truckee. Try the ceasar salad. There's a great microbrewery/pub there called 50/50.
If you are a golfer, lots of great courses. If you are a hiker or mountain biker, lots of great trails to ride, and Northstar has lifts for downhilling.
There is macanaw fishing on the lake, and trout fishing on the Truckee river.
In any event...you get the idea. Lots of things to do there. Really a mecca for outdoors people in the summer.