This sprawling city hums with the energy and independent spirit that turned it from swampland into the fourth-largest city in the U.S. Everything is big, from the towering skyscrapers, to the huge sports Astrodome that links with two Six Flags theme parks to create one gargantuan entertainment complex, to the city's prominence in aerospace, oil, shipping and finance. Even the architecture makes big, bold statements, like the medical center towers that resemble two giant syringes. A thriving business center, the city has become increasingly cosmopolitan, with an influx of diverse ethnic groups and a strong emphasis on the arts. Opera, ballet, symphony and theater are all top-notch, and museums abound. The midday heat is easily escaped inside the numerous attractions and shopping areas, particularly in the pedestrian-friendly 6.5-mile underground city. Since you're in the place that broadcast man's first step on the moon, you may want to pay a visit to Space Center Houston, where you'll be whisked into outer space through simulations and films. Once back on earth, you can hop on the tram to the complex's NASA/Johnson Space Center to watch astronauts and engineers at work and in training.  More info