I have a 17 hr layover in BeiJing. Is there a place I can leave my luggage and go sightseeing.

Don't worry! You can find the Left-luggage Counters at T1, T2 and T3 to store your luggage for the sightseeing.
Here are the addresses of the counters:
T1, on the east of Corridor between T1 to T2
T2, near gate 9 in the arrival hall on the first floor and gate 10 and gate 8 on the departure hall on the second floor.
T3, East of public space of Arrival Hall at 2nd foor of T3-C and West of public space of Arrival Hall at 2nd foor of T3-C.