I'm a bit partial to the Marriott Grand Residence Club, but that is where I own...
In my opinion, the best time to summer time visit is early June (before about June 21) and then after about August 15. Mid-June to early August is peak summer time.
If you are going in the off-season, check the schedule for the Heavenly Gondola. Sometimes it only runs on the weekends. But, it is a spectacular ride.
Take the MS Dixie II or the Tahoe Queen for a day-time lake tour to Emerald Bay. It is a great way to see Emerald Bay.
Drive around the lake. Make sure you stop at some of the overlooks around Emerald Bay, in Tahoe City, and at Sand Harbor Park (on the east side)
In the Heavenly Village complex in South Lake Tahoe (where the Timberlodge and Grand Residence are located), check out Fire+Ice, Base Camp Pizza, and Driftwood Cafe.
If you golf, the Edgewood Golf Course is beautiful...
The casinos can be kinda fun. Though, for me, "big losses" is $10.
Harvey's has been lining up some impressive summer time entertainment in their outdoor arena. So, if you are a concert goer, do a search for Harveys Tahoe.
If you go in the summer and want a bit of culture, they do a Shakespeare play each year for about a month at Sand Harbor. Search for Tahoe Shakespeare Festival.
Here is the South Lake Tahoe Visitors site:
http://tahoesouth.com/
A day drive to Reno (about 2 hours) might be fun. Also a drive down to Carson City can be nice.