Most of the wonderful trails in Point Reyes National Seashore are off limits to mountain bikes. There are several that you can ride that are decent, but the majority of good rides are actually outside of the Seashore itself.
It also depends on what you will be doing. If you are riding with kids along, there are a few options in the National Seashore that are easy enough for your offspring to ride with you, such as the Coast Trail or the Bear Valley Trail. If you go to nearby Samuel P Taylor State Park there is the Cross Marin Trail running along a creek.
If you are riding solo or with other adults or teens, then you have lots of options. If you want a ride that is not too far from the Seashore try the Bolinas Ridge Trail. You can combine it with several other trails to make a loop ride.
Much of the area near Point Reyes has restriction on singletrack for mountain bikes, so you will mostly have to ride fire roads, which there is an abundance.
I'm a fan of the Olema Valley Trail. There's something magical about it, and the variable terrain is interesting. Start at 5 Brooks stables and take OVT south. Out to the end and back is about 10 miles. You can also access Bolinas Ridge by taking Randall Trail or McCurdy Trail, both of which intersect with OVT. Randall is a tough climb and McCurdy is really tough.
I dreamed I ate a 10 lb marshmallow. When I awoke, my pillow was gone.
Thanks for all the suggestions. I wasn't able to do as much riding as I would have liked (kids need attention) but I was able to check out the Drake's Estero area and had a great time. Makes me want to go out that way again without the family.