Super Stardust was a first pick of mine, and it was pretty good. Better than Geometry Wars, but can be insanely frustrating/difficult at time. I bought the bundle for reasons that escape me, and I think I only used the multiplayer feature like once. If you have someone to play local coop with it, why not?
Shatter is easily a $10 game, and it being a tad under $8 makes it a screaming deal. There are nights where I would play the bonus mode just to unwind and relax...
...and if you want a fairly short game that's highly emotional and relaxing, then go get Flower. It's a really great experience, but I think the $10 I paid for it was pretty steep, considering that there's not a whole lot to do after you beat the game.
I like the Trash Panic demo, and it's a fun game for $5.
Pixel Junk games are pretty good, and most of them have local coop if you have friends/familiy to play with.