Coming into Beijing that late should at least mean no traffic into the city if you decide to go straight downtown. At least the next morning you can get an early start to walk around Beijing after breakfast.

As for sightseeing, you take the subway to Tiananmen East stop and walk around Tiananmen. See the Forbidden City in the morning and then go to a Peking Duck place for lunch. Maybe you can squeeze the Temple of Heaven but nothing else in that short of a day.

Consider that traffic can be unpredictable in Beijing so make sure you leave early enough. Some folks say the airport express is dependable although with luggage can be a pain on the subway.