Go to the gym then kill the guys then get the jetpack and fly up and keep going to you
reach liberty city

ok first go to where los santos and los venturus meets on los santos there sould be a free
way that looks like testicals go down the cliff until u go underwater and u see a rock
with a starfish by it u should drop if u dont keep trying

ps.u can spawn anything down there i reqamend the quad bike 

i will post more cheats in 2 days

by the mariana ig you go under the bridge and you continue on the backside of the island
you will have your choice of boats

the cheat code is about spwan a ufo ok now here is the cheat code       L1 R1 square  R1 
right R2 r2 right L1 L1

Press left down down up up square circle triangle r1 r2

p.s this cheat might work i have not tried it yet only just found it.