Enter a safe house and find the mission replay board at the far right side of it. Activate the following cheats by using the letter magnets to spell out the corresponding codes. The letters will return to their original position and a message will appear at the bottom of the screen to confirm correct code entry.
Go to the bottom right corner of the first island (where Mikhail Faustin's house is in GTA 4). Go all the way down to the bottom until reaching the last house, then go three houses to the right. Go into the back garden to find a Go-Kart. Note - If it does not appear, go to the front door, and return and it will appear if you are at the correct house.
There is a Wet Ski mini-game called "Wetscapade" hidden beside a small dock between Lancet and Colony Island. Find the drug dealer near the water's edge in Lancet. From his position, start walking towards the water, and you will practically fall onto the vehicle.
As you approach a toll booth on a bridge, exit your car. If done correctly, your car will glide through the booth. You can then get back in the car without paying the toll and not getting a wanted level.
Unlock the following missions by performing the corresponding tasks:
HOW TO UNLOCK
"Deadly Xin" mission
Unlock the Diamond Pillbox, Golden Binoculars, Jeweled Bong, Jeweled Key To The City, Platinum Syringe, Silver Titanium Briefcase, and Wooden Spoon trophies. Find the two hidden Lions of Fo. Register online at the Rockstar Social Club then sync your DS with your account through Wi-Fi. You will eventually get a mission email from Xin.
"Xin's Of The Father" mission
Complete the "Deadly Xin" mission and have no wanted level.
To easily complete this mission, first get a police car, and log onto the police computer. Search for an arson crime (fire making criminals). Do the Vigilante mission as usual, but when you kill the criminals, pick up the flame-throwers. Start the "Store Wars" mission, and quickly park two cars at each end so it makes it difficult for the vans to get past. Let the vans drive towards you (or you go towards them). Equip the flame thrower, and hold A for approximately two seconds. The vans will explode very quickly.