Well, didn't played that much GTA V, but here we go:
1) Heists are on main missions/side quests. But you can do it on GTA Online, so that is my recommendation if you just want to go and do a heist/race.
2) Stock market. A lot of history missions will make you do something that will change the stock value of a in-game company. Knowing that, you can play with stocks (you can use it from any computer. Available after the Life Invader mission). This guide is nice too: http://www.ign.com/wikis/gta-5/How_to_Make_Money_in_GTA_5.
3) Do the Trevor side missions, they are great. The ones with Michael and his son are good too. I like going out with Franklin and Chop, only because whe you got in a fight, the dog goes crazy and attacks everyone that attacks you. My last recomendation is to watch TV in game. Every TV passes some crazy and funny cartoons made for the game that satirize other games/persons/politics. Sometimes I stay hours watching them.








