Minzu Road, just off the Liberation Square, has a cool mix of older and newer buildings and is fun to walk down.
There's the standard fast food and Starbucks, but there's also cool old-style Chinese malls that will probably start dying out soon as American-style malls gain popularity. Didu Plaza and Didu Clothing City are two good examples. They're less "malls" and more indoor markets, where just about anyone can rent a claustrophobic space. I've been to malls where you have to wait for one customer to leave before you can go in, the space is so tiny.
I'm probably not making them sounds great, but they really are. You can wander for a whole day and only hit half the floors, and you never know what you'll find. It's a bargain hunter's dream, where you can find a lot of surplus or grey market items.