I thought you already had two 360s Andy? Games are tied to your gamertag (so they can be played on any console if you are signed in) and to the console you buy them on. There is a transfer tool to switch the licenses to another machine, though I can't find the link right now.
There's not much I would get but two spring to mind:
Geometry Wars 2 (super addictive)
Outpost Kaloki X (if you are into Theme/Sim-eque games)
Of course try the demos first!
Alternatively splurge it on DLC for Rock Band/GH3.
