I've put in about 25 hours and I'm only 40% through the story, so I explore and play minigames a lot between missions, meaning that by the time I'm done I'll probably run into the same issue as you, but still that's 50 hours of content, and then GTA Online comes out which I'm looking forward to playing as much as I looked forward to playing the single player.

I'm at 80% completion right now, beat the story earlier last night. Obviously completing the main story and strangers/freaks quests will mean you're done with the most entertaining content, but I think it's important to note that doing so took me around 45 hours. Granted, I was also goofing around a bit here and there doing other things, but still....it's a big game.

Also, I personally really enjoy a lot of the side activities. Then you have tons of collectibles to go after if you're a completionist (which I am in games I really like), and you can go back to replay any mission to improve your score to get gold medals and beat your friends. I was very disappointed in GTAIV and it's lack of content besides the drawn out main story, but I have no such complaint with GTA V.

I would change 2 things about this game if I could. Let me play music while on foot, and make my own mix tapes/playlists (as in SR4). I mean hell the smart phones can do everything else right, why not play music? And second, I would have loved the ability to initiate optional heists against certain stores and companies (this would also make the stock market more interesting).

Spoiler:

You already collected every page from the murderer? Or found every UFO part?

What amp said

That Sounds like a Rockstar game. Thats why I do as much as possible while I'm still stomping through the story.

I'm @16%, I am just having fun doing random things for hours before going all out on story mode.

What will the multiplayer allow that the game itself doesn't in single player mode ?

Is there new mini games, new parts of the map, missions and such ?

No, this is amazing butt...

Yes to everything besides new parts of the map, however GTA Online will be expanding in size/number of cities/etc etc as time goes on. 

I'm only %45 through the game. I could never get bored of a gta game. I can always find some stupid thing 2 do. Did you collect every thing,without a guide. I promise you that takes forever. I plan on putting at least a 100 hours into the game and exploring every inch of the map,that alone will take forever.