Of course, you can't tackle it all in a day, but you can definitely head through the Presidio and north over the Golden Gate Bridge to the Marin Headlands – further, if you're mountain biking. The website has an interactive map with planned hikes, indicating their difficulty level and duration. You can also try a guided hike with one of the experts from Urban Hiker SF, a great way to get a bit more local insight.
Catch the ferry back across the Bay, stopping at Alameda Island first. It offers a glittering view of San Francisco across the water, but the main reason to visit is for a tour and tasting at St George's Distillery; they put the first American absinthe on the market after the ban was lifted in 2007, and also make some other stellar spirits. Try the Terroir Gin which, as its name suggests, reflects the landscape around it, with strong notes of fir.
Rebecca flew to San Francisco via Reykjavík with Icelandair on their inaugural flight; flights from Heathrow have a lead-in price of £447.80, and passengers can opt to stopover in Iceland for up to seven days for no extra fare price. Rebecca stayed at Hotel Marina Iceland in Reykjavík and Hotel Zeppelin in San Francisco. Find out more on the San Francisco Travel website.