San Diego -- Tijuana

Synopsis: This is a no-brainer. Take the 932 or the Blue Line to the border, then walk across. At the San Ysidro station, you'll see an escher-esque ramp across the rails from the McDonald's. Just follow the tourists and the Mexicans into the land of the free. $2.25 if you took the bus, or $4.00 for the trolley. Crossing the border is free, but don't be carrying anything bigger than a shoulder bag or you might subject yourself to a search by the Mexican border guards, and possibly import duties. Of course, if you cross at a really busy time of day you can get away with more stuff without much risk.