I enjoy parks. They give people a chance to unwind the middle of a hectic environment and they provide the necessary greenery needed to feel a connection to the surrounding world. Nowhere is this more true than in China.
In almost any city in China, avoiding the inevitable "bumping into people" -frenzy is quite impossible. People will push you, they will pass you, you will be surrounded by them and personal space is like the starry space above; too far to reach. While I do enjoy big cities and the pulse of a vibrating metropolis, a breather is definitely a welcome break ever so often.
When in Beijing, one has quite a variety of parks to choose from. I went to many. However, one seemed to top the others as far as I'm concerned - the Bei Hai park. Bei Hai used to be an imperial garden and it shows. Grandeur, vast open spaces and beautiful nature is all around. The area is big, almost 70 hectares, so be prepared to do some walking. If walking doesn't rock your boat, get the real thing. In Bei Hai, there are plenty of rental boats to choose from and the prices won't make you search for questionable ways to fund the remainder of your trip.
As for the crowds, well, they're definitely there. However, they seem to be packed mostly in and around the actual sights within the park. Walk a bit further and it should be smooth sailing. At least in relative terms.
If I ever return to Beijing, I am going to go to Bei Hai again. I might even bring a fishing pole and join the locals in an 7-hour "I crouch and spit constantly while I wait for the fish to get hungry" -marathon.