Out of Tahoe city, we recommend the trail to Squaw Valley along the Truckee river on a designated bike trail – an easy three hour ride for any level of rider. Also the ride following the West Shore of Lake Tahoe to Sugar Pine point and back is also very popular among cyclists – again on a designated bike path. If you wish for some off road riding you can cut into the Tahoe Nordic center and follow some fire roads in the back woods then cut back down to the lakeshore.
In South Lake Tahoe, the ride form downtown to Camp Richardson and Kiva beach, one of the best on the lake and bring your swim wear. It is on a designated bike path and very popular with many quaint establishments to have a bite to eat or drink along the way. Secondly from anywhere in the greater South Lake Tahoe area and Meyers region a ride to Nevada State beach and a bit north is fantastic. Again bring your beach gear.
We also recommend the Fiber Board Freeway which runs from Tahoe city to HWY 267 in Kings Beach, a combination of on and off road trails on a single car width fire road overlooking Lake Tahoe. There are many fire road off shoots to Watson lake and the back side of NorthStar Resort from the paved trail that one can explore and return to the original starting point.
For more information on trails consult https://www.mtbproject.com, an excellent resource for novices and experts alike.