Lots of lava rock. Intermediate to advanced. Middle trail is going to be challenging for beginners but it's do-able. Well maintained by the mountain bikers in Chico. North Rim trail follows the fence on the north end of the park boundery, a good climb to Bobby Seals Trail (or sometimes known as Live Oak Trail) which drops down into the valley. Easy to see where you are as there are good views of the valley below. Live Oak connects to Middle trail. There is a "Lower Trail" ( parallels the dirt road) with other trails connecting all three,An easy out if your looking for one.
The best is South Rim Trail, advanced riders like it. It's not easy to find. Located on the south side of the Chico Creek where Centennial Ave meets Chico Canyon Rd. Recommend getting a map of the Chico area.
Both North Rim and South Rim Trails will climb 800 to 900 feet from the center of the park in about 12 to 14 miles, out and back.
Chico is full of bike shops. North Rim Adventure Sports 530-345-2453 (Trek, Santa Cruz, Gary Fisher) Chico Sports LTD 530-894-1110 (Specialized) the biggest bike I've seen. Maps of the park are available at these two shops. There are 9 other bike shops in town if you want to find 'em. Also Sierra Nevada Brewery is on 20th street and Chico Brew House is on 250 Cohasset Rd. 894-2149, (my favorite).
I don't need to tell you to bring lots of water. Summer temps can get to 100 deg.
Winter rain will close the trails.
Tred lightly, ENJOY!