What you can do in gta 4 if you get bored completing the story:
1- Record a loquendo video, youtube is folded of them and it's sooo funny
2- Go to all the venues in Liberty City: restaurants, bars, billiards, strip clubs ...
3- Meet your friends or your girlfriend for a drink or go bowling, billiards.
4- Try to drive and behave like a good person (this is going to be really difficult).
5- Carry out a police massacre. I recommend that you go up with the helicopter trick to a not very tall building and start to spar, from that place you are protected and you see everyone, the only thing that you be careful with the helicopters.
6- Explore Liberty City, for this I recommend that you go on foot.
7- Help the policemen by connecting to a police computer in their cars to search for the current crimes 10- and kill the criminals.
8- Also accessing a police computer you can find the most wanted and kill them and their gangs to be the only criminal.
9-If you have gta iv for the computer, entertain yourself with its mods.
10- Combine the tricks. For example, being in the helicopter if you have the health trick you will never destroy collisions with what you collide.
11- KILL everything that moves !!!!
12- Find all the mysteries of Liberty City, like the heart of the Statue of Liberty.
13- Do jobs for Brucie and Litell Jacob.
14- Make handsome moves and drifts.
15- Buy all the Liberty City clothes !!!!
16-Put some good milk.
17-Enjoy the things that your friends give you: weapons, bombs, thugs ...
18- Look for videos of mysteries on the internet and try to find them.
19 Try 100% of the game.