You can either pay $5 at the parking lot, $100 for an annual pass, or nothing at all if you don't mind walking an extra quarter mile (you're there to walk anyway, right?). Most people just go up to the waterfall and back, so that's the most crowded trail. For something a bit more challenging, hike up Sunset Ridge where you climb 1000ft in the first mile! The view at the summit is beautiful! If you want to keep going, a little ways further there's Skull Rock where you can stop and look around. Another high-point is the radio relay tower, where you can look out and see downtown LA, Santa Monica, and even Redondo beach. That trail keeps going and is known as the Temescal Ridge Fireroad, and it's LONG. I've been to the junction which is almost 8 miles from the Gateway and have had to turn back from the heat or running low on water or whatever. At the Junction you will find a bathroom but there are no other facilites along the trails; only back at the Gateway. I absolutely love it here and the further you go in, the less crowded it gets.
Pros: views, trails
Cons: arrive early for parking