Sadly there's not much to do. Each character's story mission combined is shorter than GTAIV I believe. Other than causing chaos (which ends very shortly when the police kill you) there's isn't a lot of fun activities to do on the side.. unless you like playing tennis or doing yoga.. in a game about being a criminal that is called "Grand Theft Auto".
*Sigh*
Don't get me wrong, its by far the best GTA in terms of technology. The story was good. The gameplay is improved/fun. The map is satisfying enough. What made it disappointing to me was the over hype. I was expecting so much more after hearing the raving before the release. The game definitely gives off great first impressions.
However, the flaws become over baring to me after what I was forced to expect. The gun supressors are useless! They actually decrease stats and people react to the gunshots from blocks away. Most of all, I just wish you had fun, sadistic, sociopathic, criminal things you could do in the free roam. I can't do anything without getting the police after me. Its like they don't want you to have fun. Outside of the story, there isn't too much ways to make money. There's properties with weekly incomes.. but since the only way to greatly pass time is saving in your bed, it would take actual days worth of saving to build up on the incomes. There's the random events that can earn you small amounts, which are nonreplayable. You can rob gas stations but I got $20 once lol. Also, the scrapyard is a decent way to back money. You get $500 for each car destroyed.. however, you pay $5,000 if the cops kill you in the process so it would be counter productive.
Another inexcusable issue is the car saving. It works perfectly in online, your car will always be in your garage when you get back to the game, destroyed (which can be insure) or sold. However, in story mode, your car will stay in your garage but god forbid you joy ride because you'll probably never see it again. So if you buy two D-Types and lose them, you'll lose all your money and have nothing to show for it. Unlike online, you can't insure it, it doesn't show you where you left it, the last two cars you were in will end up in the pound (only the last two) and there's no mechanic feature. Why they couldn't incorporate the superior online car features in to story mode is beyond me. Unfortunately, the only way to keep the cars you want in your garage without having to worry about losing them is by not saving during your freeroaming sessions.
Just thought I'd share my thoughts because I have mixed feelings about this game.