It was confirmed in one of the IGN articles that tennis, golf and the triathlon will be mini games. It sounds like they are quite extensive too, with a full round of golf able to be played, although they won't be a large part of the game in general.
It was also confirmed that each character will have their own games or jobs available. The ones above sound like they might be suitable for Michael.
I can see the inevitable car collection task falling to Franklin.
There will also surely be racing - maybe each character would have an advantage in a particular type of race, for example, Trevor being the better character to use for any flying races. This could probably apply to any of the games I guess.
Animals are also in the game so perhaps there will be hunting or dog fighting (Franklin and Chop).
I would like to see some old school video games, arcade and console. Nothing too involved, perhaps the ultimate easter egg of a condensed version of the original GTA on one of the consoles.
Also something like the amazing race, where you have to get to the far corners of the map using different transportation and doing a few small tasks.
Gambling will surely be in it, whether it is card games, slots or sports betting. A few pub games would be sweet too.
Can easily see Trevor drug dealing or committing petty crimes for cash too.
So, going on the information we have, what else can you see in there??