I had two weeks relaxing in Tahoe last Sept/October. Sunny and very warm 1st week, snow in the second.
Definitely go to a bike shop - I spoke to the guys in Olympic bike shop, Tahoe City. He spent 30 mins highlighting all the best local trails on a map, whilst my wife got sunburn outside. I felt so guilty for taking up his time I bought a water bottle - although the Trek EX8 in stock looked like a good deal...
Map is called: 'Lake Tahoe Basin Trail Map' - there are literally hundreds of trails.
Tahoe Rim Trail, Mr Toads, Armstrong Connector, Flume Trail all on the map.
Best ride for me was from Tahoe Meadows (located NE on the lake, on the road pas past Incline Lake), south on the ridge, down the Flume Trail and back around Marlette Lake (located half way down on the east shore of Tahoe). Really amazing scenery. There is also a 'Tryolean Downhill' trail which people were shuttling. Looked fun, but have a small todger and a hard tail, so too much of a big boys ride for me.
Also Armstrong Connector up to Armstrong Pass, back down connector and Cedar/Corral a real blast (on the descent). Climb is up to 8,500 ft though. (SE lake).
We stayed NW of the lake - south lake is busier, more night life and a bit more touristy.
Yuba expeditions were really helpful on email advice, but I never made it up to Downievile - trails looked very good.
Last bit of advice would be pack a spare set of lungs - the climbs were tough, but the views were awesome and after 7 days of riding trails/road (mixed up to keep the wife happy), I was a lot fitter on return to the UK. Would go back (but would try and buy a full susser by then).