For people still wondering and creating a billion threads in search of answers, here's what we can do in free roam regarding Hobbies and Pastimes (from BradyGames Guide):
Arms Trafficking Air
Arms Trafficking Ground
Bail Bonds (Bounty Hunting) - Trevor Only*
Hunting - Trevor Only*
Property Management (side-missions available every few days ranging from; Whack this person, Steal this, Deliver this, Rescue him/her or race/chase that car)*description by TheRealBars
Sports: Darts, Golf, Tennis, Triathlons.
Armoured Van Robberies
Scraps of a letter(50)
Nuclear Waste Collection
* 49 Random Events (mini-missions, such as ATM Robberies, that may reappear, although in the guide they are numbered, such as Mugging 1, Mugging 2, etc.) If you have ever played Red Dead Redemption, you are aware of what I mean by Random Events.
*Storyline Missions: 69 Missions. There are 82 Missions listed in the guide, which most probably represent the main missions broken down into every sub-mission.
Other things people are mentioning:
Knife Fights - mentioned by iNero
As you can see, there is no casino gambling included in free-roam. Cinema is also not included in the activities, although the Cinema icons do appear on the map, which means there are Cinemas, but they are not considered as part of "Hobbies/Pastimes".
*Thanks to TheRealBars for filling in some of the things I missed.
Edited by DobertReniro, 15 September 2013 - 11:11 PM.