You can swim, you can buy/change clothes, you can go on dates, eating replenishes your health. That is about all that carried over from VC/SA. Instead of focusing on yourself, Rockstar said they wanted to focus on the relationships between Niko and the other characters. So instead going to the gym during your down time between missions you call up one of you friends to go out drinking, eating, bowling or a number of other activities. These people may also be POCs for certain side missions/jobs that allow you to mess around and/or make some cash. When your relationship reaches a certain point you will unlock that person's special ability that you can use.

From what I have seen so far and heard from others is that there is no longer any real estate available to purchase nor are there parking garages for your cars. There is a parking spot outside of you apartment(s) where you can save a single car.

The cop mode has changed to a search radius that increases depending on your star level. While sometimes a bit easier, I do find it more enjoyable, especially with 3 or more stars. The search radius recenters on you anytime you cross a cop's line of sight. Once outside the radius and no cops are in sight you will have a brief cool down period before your star level is gone. Right now, it seems that the cool down time is a bit too short.

Im still on the first two boroughs but I would definitely place the size of Liberty City between VC and SA. But while the square mile is smaller than SA, they have placed a hell of a lot more detail and polish into the smaller area. So far from what I have played, I say its better than San Andreas. If you liked any of the GTA3 series of games, you will like GTA IV.