The Flume Trail is one of Lake Tahoe's premier mountain bike rides. It has been featured in mountain biking magazines and can get pretty crowded on summer weekends. You'll get a good workout getting up to Marlette Lake from Spooner Lake, but once on the Flume, the views will quickly erase any memory of the climb.
There are several variations to the Flume Trail. By far, the most popular is the one-way Spooner-Marlette-Flume-Tunnel Creek ride. This ride is 13 miles long, climbing 1,200 feet and descending 1,800 feet.