Japan -> Bejing(stay 2 full days) -> Hong Kong

So we are planning to fly from Japan on US passports to Behing, spend a couple days to see the great wall, then fly to Hong Kong and board a cruise that ends in Singapore. Can we take advantage of the 72hr twov in Bejing and then just to confirm...We don't need a Visa for Hong Kong. Also, would it be possible to do Japan -> Bejing -> Shanghai -> Hong Kong?