The economic revival sweeping downtown Las Vegas has ushered into the area restaurants, bars, entrepreneurs and residents. High-rise towers are filling up with renters and ground-floor tenants. Bars are taking over vacant buildings. New restaurant ideas are being hatched, and tech startups might soon outnumber casinos. Much of the activity is centered around Zappos, the online shoe and clothing retailer moving downtown from Henderson late next year and bringing with it hundreds of young, well-heeled employees.