Mutianyu is the best all-round choice for a first visit: beautiful, with a cable car, and less crowded than Badaling.
- Mutianyu: about 1.5–2 hours from Beijing; cable car up and a toboggan down.
- Badaling: closest, reachable by high-speed train, but extremely crowded.
- Jinshanling and Simatai: wild and scenic, for keen hikers; further away.
- Take a tour, or hire a private car if you're a group.
Note: avoid the National Day holiday in early October and Lunar New Year, when domestic crowds are huge.