Finishing Halo 4 will give you cash back on your credit card. And it's not like it's some $1.52 bullshit you get back from Discover either, it's $25 off (or £25 in the UK) your American Express bill, provided you link the thing through a promo site.
Linking your card and collecting the "Wake Up, John," achievement-granted when you finish the game's campaign-delivers the rebate to your card. This is the first I have ever heard of a game giving you cash off your credit card bill for unlocking achievements. But AmEx and Microsoft have done the deed, so you might as well take advantage of it, if you have the card in your wallet. See here for more details (here if you live in Great Britain.)
Just linking your card with your gamertag gets you $10/£10 back on your statement. Completing a Spartan Ops mission (and getting the achievement for doing so) on any difficulty gets you some kind of coupon dependent on buying $100/£100 worth of stuff at a particular retailer.
There's also a grand-prize competition: Be one of the first 25 in the U.S. (or first 5 in the U.K.) to finish Halo 4 on Legendary Mode and you get a free trip to E3. Oh, Christ. Having gone to four, let me tell you, second prize is two trips to E3.