PLAN YOUR TIME HERE...This is not a "hey lets go over there for the afternoon kinda place". And if you can, try to get your tickets for the tram or segway tour way in advance.
Took the ferry from Oakland which goes through the San Francisco terminal; neat because you pass under the Bay Bridge and go past Treasure Island and Alcatraz. Note: you can camp overnight but need to plan in advance. Head towards the main park where you can take a tram tour of the island, and now Segway and scooter tours. A lot of locals bring their bicycles. The road that goes circumnavigates the island is paved and well maintained.
We took the first ferry over and then foolishly hiked up the path just left of the dock when you get to the island. This is all stairs and I would only suggest that advanced hikers/walkers take this route; it climbs about 400 feet in a short distance. After huffing and puffing my way to the top we set off for the immigration museum. Another hike downhill led us to an amazing set of buildings where we spoke with one of the docents for some time and toured the exhibit. Two hours later we hoofed it back up the hill and headed back to the park where we set out the blanket and enjoyed our picnic lunch. We had tickets for the tram tour around the island because we wanted to see a little more of the area.
The ferry ride back at the end of the day was nice but the fog was coming in pretty quickly so it was colder than I was expecting. Ferry ride tickets were roughly $30 for 2 people. No fees to gain entrance to the island. Picnic lunch and a couple extra bottles of water $20 from a deli in Berkeley. All in all, pretty priceless day in the SF Bay Area. We'll be back.