If I had one day to take a newcomer to San Francisco, this is what I would do.
Assuming it is a clear day, and that is a huge assumption in San Francisco, I would start at the beginning of Market Street. If you look toward the financial district and look all the way to the end of market, you will see Twin Peaks and the antenna. The top of Twin Peaks will be your goal for the morning.
Start by driving up Market Street. You will stay on Market Street until you get to the top of the hill at Twin Peaks. Before you think about the final destination, you will be driving past many landmarks and I would recommend you stop the car and park to enjoy the sights and the feel of each of the neighborhoods. I would drive through the Financial District unless you enjoy seeing skyscrapers. If that is your thing, stop and enjoy them. You can walk to the Transamerica pyramid in 15 minutes from the Ferry Building. Keep looking up and follow the pyramid.
Once you pass Kearny Street on Market, you will be passing in to the Garment District. Please stop the car here if you want to do some shopping. If you stay on the right hand lane and veer to the right on to Geary Street, you will find the Union Square parking garage 4 blocks away. Park here and enjoy the sights of Union Square. The shops to look for are Macy's, Tiffany's, Nike Store, Levi's and Neiman Marcus.
Once you are done there, get back in the car and try to get back to Market street and make a right hand turn to continue going up to Twin Peaks. After the Garment district, you will be in one of the worst parts of San Francisco. This is called the Tenderloin. Although they have some great ethnic restaurants in this part of town, I wouldn't recommend any tourists stopping here. It is too risky. Look to your right after crossing 8th Street and you will see San Francisco City Hall. It is an impressive dome structure. Be careful of stopping here. It is still a fairly unsafe neighborhood.
If you continue up Market Street, you will be in the Upper Market neighborhood. Keep following Market Street and when you pass a street called Church Street, you will be in the Castro. This is the famous alternative lifestyle neighborhood. It is a vibrant place with a number of great restaurants and shops to go to. I would recommend parking the car here and taking in the neighborhood at the Peets Coffee on Market Street.
When you are ready, head on up Market Street until you get to the top of Twin Peaks. While you are driving up, remember to look to the left. There are spectacular views while you are climbing the mountain. You will go all the way to the top and come back down again until you have to make a right on Twin Peaks Blvd. Follow this street for about 2 miles or until you get to the lookout point. You can't miss it. It has a parking lot and telescopes so you can put your take a closer view of the bay from the tallest peak in San Francisco. On a clear day, it is truly breathtaking. If you stand in the right spot, you can see where you drove up on Market Street.
When you leave, make sure you make a right so that you are on the other side of the mountain. Keep going down until you are on Lincoln Blvd. This is the main road that can take you to the nearest freeway.