I've ridden across the US twice, once in 2001 and again in 2006, and visited both Vegas and San Francisco. LV? An amazing, surreal place created my mankind after WW2 out of nothing. Baking hot in the summer. Cheapest hotels in the US as each has it's own casino which subsides the room costs. I stayed in Circus Circus (hotel) each time which is vast and cheap. Many people visit LV and never leave their hotels, prefering to pump money into slot machines and play the tables. The town doesn't look much during the day but comes alive at night with fantastic fountain displays (Bellaggios), real life pirate ships and battles (Treasure Island), replicas of places such as the Eiffel Tower and one of the the world's most famous hotels, the Venetian with it's indoor canals and gondolas. Two days was enough for me and I needed to get away and my mid-afternoon I was in Death Valley (118F and 100F at midnight, would you believe). My wife flew out and we did get a heliopter flight into Grand Canyon, landing near the bottom and enjoyed a champagne breakfast. A big adult's playground.
San Francisco. After the heat of the desert, the cooling breeze of the Pacific is a relief. Lots of places to walk and see - the Coit Tower, the seafront, boat trips to and around Alcatraz, and more restaurants than you could shake a stick at. My 4th favourite US city after San Diego, Boston and New York.