Located on the west side of the San Rafael/Richmond Bridge, on San Francisco Bay, this park is extremely popular with mountain bikers. There are definitely better mountain biking trails in Marin (Tamarancho Boy Scout Camp, for example) but keep in mind that the bar is very high for mountain biking in Marin. So China Camp won't disappoint even if it's not the most epic biking in the area. The trail running and hiking is great too. The campground is off the main road that parallels the bay. It's located just past a meadow, and in the (increasingly rare) wet months, there's a creek that runs through the park. The website will say the sites are "hike in", but this is not how I'd describe it. Yes, you need to park in the main lot and then carry your gear to the campsite, but it's not really a hike. More like a short walk, a quarter of a mile at best. This means you will be able to bring your fully iced Yeti loaded with tasty local microbrews to your campground. Heck, maybe you can figure out how to hitch it to your mountain bike so you can tow it in.
The park itself is classic country California - oak, madrone, and even the occasional redwood. It's often foggy in the morning, and sunny in the afternoon. You can camp here year round, and I've been here in the winter, spring and summer. I really enjoy it here when the creek is running, but even in the summer when its dry, the proximity to the bay means it probably won't get too hot. Bring your mountain bike and shoes suitable for hiking and trail running. I've never done it, but I'm pretty sure you could kayak in the bay here as well.
This park is 4 stars because one would never plan a trip to visit China Camp on its own, but it definitely would be included as part of a larger trip to Marin county. A great Marin trip would be: set up camp at China Camp, day trip to Muir Woods, day trip out to Point Reyes, half day of mountain biking in China Camp, full day of mountain biking on Mount Tamalpais, full day of mountain biking at Tamarancho. Drink plenty of local microbrews in between. For locals, this is an awesome spot located right in the Bay Area. I come here often, and I've never had a bad time.