The roughly four-mile-long Barbary Coast Trail preserves over 50 sites from the region's more rambunctious days during the California Gold Rush. It was home to one of the most notorious red-light districts ever with saloons, bordellos, gambling halls, and dance halls on every corner. This little area of sin managed to escape the wrath of the 1906 earthquake and is now the site of the oldest commercial buildings still remaining in San Francisco.
A popular way to do the whole, almost 4 mile length of the trail is to start at one end and then take the Powell-Hyde cable car back to the either of the starting points. The trail is marked by 170 gold color markers implanted in the sidewalk, each with arrows pointing to the next significant spot. You will not get lost, but you may find yourself going in circles from time to time. That's part of the fun.
Pick up the trail at Beach and Hyde, a block from Ghiradelli Square or downtown at the Old US Mint on 5th and Mission and start weaving through historic and always interesting San Francisco neighborhoods.