Lassen Volcanic National Park

From: http://www.searchpictures.net/

Congress established 166 square miles of wilderness at the southern tip of the Cascade Range, 50 miles east of Redding. Its lava domes, waterfalls, and forested lakes are as breathtaking as the beauty of Yosemite, yet it is not overrun by tourists (not even 1/10th of the visitors!). The top draw is the hike through the giant lava tube, 700-foot high cinder cone, and steaming lake.

Come July, when the snow melts, the national park's amazing geology is supplemented by wildflowers that explode across the alpine meadows.

Mount Harkness offers awesome views and a ton of wildflowers (better than Lassen Peak).  Bumpass Hell (With a name like that? Who would want to skip out?) is a 16-acre cauldron of steaming ponds, steam vents, and boiling mud pots. Yellowstone Jr!

Start the day in the Kohm Yah-mah-nee visitor center and end it on sunset at the Pilot Pinnacle.

No lodging within the park, though.