I don’t think you need to go through a travel agency at all—Tijuana is right across the border from San Diego, and it’s only about a three hour drive max, so you can just hop in your car and go. The border crossing is very easy on the way there—you zoom right through—but the way back can be a looong wait. If you’re renting a car, make sure to ask very clearly about their policy for taking hte car to mexico. Some companies can track the rentals and you don’t want to be slapped with any crazy fees. Incidentally, not sure if this is your first trip down there: Tijuana is a crazy big city, but if you drive down the coast an hour or so you get to some lovely beaches.