Here's a new way to do self guided tours of a few cities... San Diego, San Fran, Miami, Barcelona, and Lisbon

A bit pricey to me, 1st hour $49, 2nd hour $39, Each additional hour $29

But a pretty cool way to zip around the city and enjoy the perfect San Diego weather, a Time Magazine "Invention of the Year", and the typical GPS guided tour is an hour and a half: http://www.gocartours.com/sandiego/sandiego-tours-downtown.html