3.5 miles, Half day
The Summer Palace was the retreat of emperors for centuries, gradually enlarged and repaired after taking beatings in the Second Opium War (1856-60) and the Boxer Rebellion (1900). It's now one of Beijing's best-known tourist destinations, so in the summer it's crowded with visitors looking to catch a breeze around Kunming Lake. The palace is usually empty if you visit in winter, but the biting cold is the price you pay for uninterrupted views.
The Summer Palace is really easy to get to by subway, the cheapest and most tourist friendly way to get around Beijing.
Don't be intimidated by the subway in Beijing—everything (including announcements) is in English and Chinese and is well-marked. Try to avoid rush hour as trains (especially on lines 1 and 2) will be very crowded. It's about a 25-minute journey from Xizhimen station where Line 4 crosses Line 2 as a rough guide.
Using the subway ticket machines is pretty intuitive. The touch-screen button for English is in the bottom left corner of the screen. Select the line you need and touch the station you want. You'll then be prompted to confirm the number of tickets and the amount you need to pay. There are maps both on-screen and on the walls to check which line you need.
Ticket machines in the subway don't accept 1RMB notes, just the coins. (Renminbi Yuan—usually known as RMB or just yuan—is China's currency.) If you don't have change, or only big notes, go to a ticket window and hand over 2RMB—it's a flat fare for a single ticket (you can't buy returns) wherever you go.