I'm a little biased, but if you want to quickly get a feel for the entire scope of the game, you can go from count of Ulaid to Duke of Ulster to King of Ireland within a pretty short time. As count of Ulaid you only need to grab another province from another count in order to lay claim to the title of Duke, and then you can politik it out with your neighboring dukes and compete for vassals and eventually go to war in order to unite the kingdom.
I've done it a few times and it never fails to entertain. I like starting as a count and working my way up, and Ireland is one of the handful of places where you can do that without waiting forever.
There's also a duke in southwestern France who has a daughter you could marry a son off to(as a count in France) and secure inheritance for a son via marriage right off the start of the game, but you need to be willing to get your hands dirty and assassinate his two eldest sons. But if you do that, you can secure a big duchy and eventually break free from France, or maybe try to secure a claim on the throne for a son through marriage and then go to war.
Geez, making me want to play again!
