Red Rock loop is a must. Most people start at the Albertsons at the intersection of Desert Foothills and Charleston. The River Mountain Loop is also fun. If you start from the trailhead just before the Lake Mead Rec Area gate on Lake Mead Parkway and ride it clockwise, you've got some really fun switchbacks, descending 1500'. If you ride it counter-clockwise, you'll face some pretty challenge climbs with grades as steep as 19%.

Another popular route is from the M casino to Jean. You then take a right just past the gas station in Jean and head out to Goodsprings which has the oldest saloon in Clark County. Grab a beer and head back. From Jean back to the M, you're in for a really nice downhill and if the tailwinds are in your favor, you should be able to cruise in at 40+ MPH.

BTW, I live here in Vegas and just did the Red Rock loop three times yesterday. I drove out an parked near the entrance so that I had a cooler filled with drinks. 3 laps=3 hours and approx 5000' of vertical.