From Mutianyu Great Wall, what's the easiest, most convenient, and cheapest way to Olympic Park?

From the Mutianyu Great Wall, you need to hire a local car to reach Huairou North Street where you can find the bus line 916 to get back to the Dongzhimen Bus Station.
From Dongzhimen bus station, you can walk to the Dongzhimen Subway Station to take the subway line 2 to Gulou Dajie Station where you can change to subway line 8 to get to the Olympic Park South Gate Station. After that you can follow the signs to go to the park easily.