I've been to Colonial Williamsburg many times. The best times to go are the first two weeks in June or after labor day. In other words, when school is in session. Parents drag their kids there during summer break - often kicking and screaming, until they see it and enjoy it. So, bigger crowds and high humidity. Amazingly, the natives in colonial times didn't have air conditioning, and neither does Colonial Williamsburg. Dress for it.
If you have the time, visit nearby Jamestown and Yorktown. They make up the other two thirds of the "historic triangle". At any of them, you can buy a ticket good for all three.
Colonial Williamsburg covers a large area. But they have free shuttle busses that drive around the historic area, stopping at several places, so you can hop on and off for your entire stay. Still lots of walking, so wear comfortable shoes.