So it occurs to me, there aren't soooo many fun driving routes easily/quickly accessible from Orange County . . . at least not so well known ones. This thread is for compiling routes. A fun driving route is something other than just a straight city road that stops a lot.
So for me, from Fullerton here's a route that would take a couple hours total and go through several canyons and a patch of PCH. Fun parts marked in Red.
5 South
55 South
PCH South
Laguna Canyon 133 North
241 South
(The last parts of the 133 and the 241 are toll roads.)
Here's the part where I have to check a map. There is a way to get in through Rancho Santa Margarita to the back side of:
Santiago Canyon Road
This comes out to Katella.
Katella
57 North
Imperial East
Carbon Canyon
71 North
60 East
57 South
Brea Canyon
Brea Blvd. back into Fullerton.