Unlock all cities:

Rename any character to either LEVELS or WENDY

Bonus characters

Rename Kelly to SUMNER

Unlimited fuel

Rename any character to FULL

Fly Mode

In Vice city go to North Banana Grove. There is a hospital with a flight of stairs on it, climb up the stairs to the top of the building. Walk off the edge of the building and you can now walk accross buildings, water etc. To go back to normal go down a flight of stairs