It's Day 2 of summer. If you don't have vacation plans yet, you're behind. Fortunately, there's an easy solution. Visit one of America's 58 national parks. They encompass some of the country's best natural sights, including Alaska's pristine wilderness, Yellowstone's hot springs, Utah's rock formations and Hawaii's volcanoes. As a starting point for your vacation planning, here's a look at the 10 most-visited in 2011.