Walk over border from SanDiego CA to Tijuana is free.
I know Tijuana probably doesnt accurately represent Mexico but it is an interesting place all the same.
Once rented a car in Tijuana and drove down Mexican coast about 100 miles or so to place called Ensenada. Nice place. Coastline on way down stunning.
If you want a good holiday I would reccomend following:
Go to California.
Drive
San Diego - wonderful place
LA OK
Las Vegas Great place
Grand CanYon Great
Baja California (part of Mexico described above)
What more could you want.