While Devil's Slide and Vivian Creek trailheads require permits, I've never had a problem getting one. You just have to fax in the permit a week or so before your hike to the Ranger Station.
Like wbtravis5152, I agree that Devil's Slide is a great trail to San Jacinto summit. About 20 miles roundtrip, climbing from about 5500' to 10800'. Much more challenging (and beautiful) than taking the tram.
San Gorgonio offers a ton of great trails---Vivian Creek from the South (steep!), and Dollar Lake or Dry Lake from the North, or Fish Creek from the North. The trails from the North areas are all about 18-22 miles round trip, but the climbs are not as steep as from Vivian Creek.
Baldy is a great hike, steep, but not as many miles roundtrip.
Have fun out there!