It was the historic capital long before Beijing, so it has tons of old Chinese Dynasty history. It has a "great" city wall (it's not THE Great Wall, but still pretty great!) that you can explore on bicycle, which is fun for kids. It has the Terracotta Warriors and Tang Dynasty Palace for history. The Muslim Quarter is great for souvenirs and food (say hello to Helen, who is a sweetheart and will do hand-painted squares with scriptures in Chinese caligraphy. She's Christian, and considers her work a mission of sorts). The Wild Goose Pagoda is cool for seeing the typical early morning Chinese morning exercises/dancing/kung fu demonstrations like they do in major parks throughout the country. We just loved it.

Beijing is cool, of course, but everything is really spread out so you end up walking a lot more.

Shenzhen is another one you may not think is cool, but it actually really is. You're just across the border from Hong Kong, which is awesome, and Shenzhen is the counterfeit capital of China, so it's awesome for shopping. Dafen Oil Painting village is the weirdest, coolest thing. Half of the oil paintings in the world, including all the fancy hotels you've ever stayed at, come from these few blocks where Chinese artists reproduce art from pictures on their phones. You can buy yourself a Van Gogh for $5! Or literally anything else you may imagine. And the Chinese Cultural Villages is AWESOME. It's a theme park that has little villages set up to represent different Chinese native ethnicities, and you can see how they lived, watch dancing, eat different food, see miniatures of major Chinese landmarks (not too miniature, still really impressive ... for example a 30-foot Buddha!). The Windows to the World theme park is the same thing but scaled down versions of major places around the whole world. See Niagara Falls, ride a camel, etc.

Shenzhen is surprising really cool.