A nice long ride on double track. Not "extreme" but worthy in it's own right.
Ride up road 2900. Whenever you come to an intersection, stay on the main road. Much of the road passes through logged or burned out areas, offering excellent views along the way. At mile 5.5, the road enters woods and ends at a couple of streams after 1.9 miles. Look for an ORV trail off to the right. Follow the trail through the wet area and around a trench. The trail ends at a gravel road; turn right. Take the left at each of the next 3 intersections. The road ends just below the summit. There is an old dual track off to the right. Turn right and spin up the switchbacks to the top. When you're to go back, just go back the way you came.
How to get there:
Take I-5 north from Seattle to exit WA 20 (exit 320). Follow WA 20 through Burlington and Sedro Woolley. Turn left onto Cabin Creek Road (17 miles from I-5). At .3 miles the road comes to an unmarked intersection, turn left. The road will come to a Crown Pacific entry guard station, where you will have to obtain an entrance permit. Stay on the main road for another 2.5 miles, keeping an eye open for the Mount Josephine Truck Trail on the right. There is a wood sign that says "Truck Trail/sw-ho-2900". Park on the side of the road goes left here.