Anywhere in the National Forest (that isn't closed due to toads or something) is legal at night. My favorite and a popular choice is the Santiago Truck Trail. Two options are 1) up and back parking on Modjeska Grade Road, 2) Park at Cook's Corner, climb the road up to the start of STT, go to Old Camp, catch the Luge on the way back down. I usually go with option 2.
Another variation is to park at Cook's, go into Whiting Ranch via the horse ranch, do that loop, then cross Santiago Canyon Rd and do the STT/Luge loop. All 3 variations are about 16 to 18 miles with about 3000 feet of climbing. Just doing the STT/Luge is about half of that.