Campaign Solo, Online for a while to see what its like, Get all of the achievments i.e co-op,online etc…, then Destroy everyone online untill Halo 4 comes out.
Appear offline, fire up Legendary Campaign and don’t stop until I beat it and find all Terminals, then I’ll get some sleep and then play on all of the new maps (especially Installation 04). Probably followed by some more campaign.
I will go on Campaign and play as much as I can before I get stuck on something, which will likely be on 343 Guilty Spark. After that Forge a bit, go on Matchmaking for a bit, go on Firefight for a bit, look at some clips in Theater Mode for a bit, then go back and beat the Campaign.
That is exactly what I did for Halo Reach. And after that, play it and some of my other games while waiting for Halo 4.