Not even Xbox LIVE's Major Nelson can pretend this is anything else -- Avatar: The Last Airbender: The Burning Earth is now available on Games on Demand, and it's the simplest way to get 1000 Gamerscore points. Ever.
The game will cost you $19.99, and that roughly translates to the easiest boost of Gamerscore you'll be able to find. Yes, it straight up involves pressing the "B" button a bunch of times. Sounds like a good use of five minutes.
SEGA Superstar Tennis is the other game available today, and that'll run you $19.99. Problem is, it'll make you work for your Gamescore points. Not cool, Sega. Get with the program.