Just beat Flynt in an epic race.
Going around tying up loose ends now, racing the events I have left and looking for 6 more discount signs. Also buying all of my favorite cars now that I'm crazy rich and upgrading them so they're fun to drive; I want to have a nice selection of cars in each class for multiplayer stuff. I'll also be trying my hand at setting good rivals times and speed trap scores.
Some of the most fun I've had in the game is doing the PR stunts, street races, and party modes online. I really enjoyed Cat & Mouse in Dirt 3, and hated it in Forza 4; it's a lot better in here than it was in 4, though it's harder to stop the mouse as you can't fishtail their car as easily. Infection is also fantastic in these little open areas rather than in 4 where you would have those slow chases through quicksand grass.
The PR photo shoots set you up for really nice shots and takes you on a tour of the nicest areas.
The F40 you win from this event is great; really responsive and just fun to drive.
I really enjoy this game. I do miss some tuning options and drivetrain conversions, but overall I may dare say that I'm having even more fun with this than 4, which I played the hell out of.