Have you not yet unlocked Liberty City’s Staunton Island or Shoreside Vale? You can get those easily, follow the simple steps below

Getting to Staunton Island: To get to Staunton Island, from Portland, you first need to use the cheat CHITTYCHITTYBB. Next, you need to find a fast car.

• If you want to hijack a police without getting caught, walk up to the passenger side, and attempt to enter the vehicle (or just take one of the cars outside of police station). Wait for the cop to get out and close his door, then you can get in and drive away. Other vehicles that are fast enough include the Banshee, Ambulance, and Fire Truck.
• Go to Callaghan Bridge. If you have not yet unlocked Staunton Island, slowly drive through one of the gaps in the barrier, then drive across the bridge as fast as you can – you’ll barely make it to the other side if your car is fast enough. If you have unlocked Staunton Island, drive across normally.
• Now to get to Shoreside Vale. Go to the northernmost docks, on the East side of Staunton Island, and hijack the speedboat. Head north towards Rockford.
• When you reach the barrier, keep heading North for a while, then turn around. There’s a nearly unnoticeable slope in the inside corner between the military base and the rest of Staunton Island. Head for it as fast as you can. If done properly, the boat will fly onto land.
• Unlike other vehicles, boats do not blow up when flipped upside down, so don’t worry if the boat flipped. What you need to do now is hijack a really big vehicle like a Bus or a Yankee. Do not go far from the boat or it will disappear.
• Drive towards the boat in your large vehicle. You may want to kill the previous occupant so he or she doesn’t get in your way. Slowly push the boat around the West side of the military base.
• When the boat is close to the water, exit your vehicle. Push the boat on foot by sprinting at it. Be careful, you don’t want it to fall in yet.
• When the boat is about to fall into the water, enter it. If done properly, the boat will slide into the water with you in it.
• Take the boat to somewhere on Shoreside Vale that you can get onto from the boat.

Note

• The only vehicles in Portland that are fast enough to get across Callaghan Bridge are: Banshee, Police Car, Ambulance, Fire Truck. There may be others that I don’t know about.
• Make sure that it’s the speedboat you take, at the northernmost docks. The other two boats aren’t fast enough.
• Line yourself up before going across the Callaghan Bridge. If you lose too much speed while trying to steer straight, you’ll fall in the water.
• Type “nopoliceplease” to make your police rating go to zero. Also, “gesundheit” make your health go all the way up.
• Avoid the cops. Be careful to never get a 2-star rank or higher.
• After hijacking a large vehicle on Staunton Island, be sure to kill the driver, or else he’ll catch up to you and attack you.
• If you have not yet unlocked Staunton Island, don’t get busted or wasted, or else you’ll end up back in Portland and have to restart all over again.
• Do not attempt this in real life.