I suggest the Powell/Hyde line - the turnaround is at Market and Powell - close to Union Square and probably your hotel!

Once the most popular from of public transportation for early San Franciscans, the cable cars have become a major tourist attraction. It's still a reliable and scenic way to travel around parts of the city. At the intersection of Powell and Market, visitors line up to get a ride on a distinct San Francisco treat--the cable car. Don't dismiss this as a tourist trap, because there is really no more enjoyable way to enjoy San Francisco's hilly views and fresh air than from one of these historic trolleys. San Francisco was the first city to use cable cars, and due to their surprising usefulness and eternal charm, it is the last city to keep them in use. Since 1873, these trolleys have been tugged at a slow but steady 9-mph cable in the ground by giant turbines. The slow pace allows passengers to take in the experience, pointing out sights along the way. Passing by Union Square up Powell, this cable car line leads directly to Fisherman's Wharf. Along the way, riders enjoy a soaring vista of Alcatraz, Angel Island, Mount Tamalpais, and the Golden Gate Bridge.