Holiday Inn Express at Forbidden City?! Are your sure? Or it should be at Temple of Heaven. This is the nearest one to the Firbidden City as I know. If so, no direct airport bus there and there is only airport buses to Xidan and Gongzhufen area around midnight. Suggest you take the bus Line 2 (to Xidan)then take a taxi to your hotel. If you take taxi direct from Beijing Airport, the taxi fare should be nearly 150yuan at that time.