My favorite trails / fire roads to ride on the 'cross bike are closed right now. I like to ride from home (foothill ranch) through whiting ranch to santiago canyon, up through the canyon to black star canyon and up to main divide. I think that it will be a while before I can do that ride again.
Another favorite ride for the 'cross bike is down the aliso creek bike trail to aliso and wood canyons park.
I've ridden the fullerton loop on my 'cross bike but I hate having to cross all the streets.