US Citizens Flying to Sydney With 12-Hour Layover

My husband and I are flying round trip on Air China from Houston to Sydney in June 2014. We have a 12 hour layover in Beijing on the way there and a 9 hour layover in Beijing on the way back. We would like to do some sightseeing during both layovers. We have valid U.S. passports and will have Visas to enter Australia. What else do we need to be able to leave the Beijing airport for sightseeing?