I highly recommend the Muir Woods as a day trip if you're staying in San Francisco. It is a great place to be inspired by what nature can create. The best activity there is to stroll among towering trees and listen to the breeze rustle the branches. It's easy to get to - just hop the ferry to Sausalito from the Ferry Terminal in San Fran and once there ask to be directed to the shuttle bus to Muir Woods.