"I was wondering: What parts of America are diverse? All of them?"

Pfft. I wish. Most places, it's like wading through an ocean of Wonder Bread.

"I need a medium-sized, diverse city that has some suburban areas surrounding it. I was thinking of Seattle."

I don't know what you mean by medium-sized, but Seattle is in the top 20, population-wise, at around 3/4 of a million. If I were looking for a diverse (for the US) city in the 500,000 population range, I'd look at Sacramento, California. Some researchers called it the most diverse city in the US a while back. It's the capital of that state, though I'm not familiar enough with it to talk about suburbs.

https://en.wikipedia.org/wiki/Sacramento,_California