Well, why don't you just take your time and enjoy the game? As well as that, try out Multiplayer.
If you finish the game early, you're going to get bored. It's a typical thing that would happen to almost everyone who finishes the game early. The developers anticipated us to enjoy the game and not to rush through it like it's a race. Most of us do take our time, but the minority of us will rush it and finish it completely, and will in the end begin saying it's a let down like you've said. So here's something; Next time round, take your time when you play IV. Enjoy it.
The multiplayer is awesome, I've seen previews of it and have actually seen my friend play it. If you play against 5-6 players, it will be fun. If you play around 2-3 players, it won't be as fun.
Another option would be to explore the city. The scenery looks awesome, so why not check it out in detail? That would be interesting to do. So basically, enjoy your time in IV, don't rush it and get bored.