When you get to San Fierro, go to the shore and travel until you see the Golden Gate Bridge. Enable the "Drive on water" and "Spawn Rhino" codes. Get in the tank and find a way off of the shore. Drive until you get to Las Venturas, and drive onto land. When you get there, you will have a four star wanted meter. Explore the city, and when the police cars hit you, they should explode. Once you are done exploring, drive back into the water or swim back. The area you just explored should be on your map. You cannot buy any property while invading the city. This trick can also be done to explore San Fierro early from Los Santos.