Yes, another topic inspired by my ladyfriend. :-)
 
As the title says, we've got roughly $25 to spend at the PSN shop, and the only PSN games we've got so far are Digger HD and Flower (both excellent purchases, I must say). We're still on the noobish side when it comes to console games, but we seem to have developed a liking for simple and/or retro-styled games. Recommendations are welcome and indeed encouraged!

You are pretty safe with any of the Pixel junk games besides pixel junk racer(not a fan of it).

Pixel junk Eden(/pixeljunk-eden/61-21181/) is a cool game solo or coop, it is basically a platformer inspired by plants and techno, if that makes any sense.

Pixel Junk Shooter(/pixeljunk-shooter/61-26101/) is a great 2D shooter, but with a unique twist. The majority of the game you are fighting enemies while also experimenting with different elements like water and lava. Like Eden, this game is better seen then explained, but it is also COOP, and a great deal of fun.

Braid - It starts out simple, yet gets deviously clever the further in you get.

Pac-Man Championship Edition DX - You can't get more simple and retro than this, yet updated and a ton of fun

Lara Croft And The Guardian Of Light - An excellent co-op experience if you want to play together.

I'm assuming your not a Playstation Plus member. I'll recommend Sideway: New York. It just came out and from what I've played it's a platformer with a neat concept of you being graffiti on the wall and that making you climb buildings in all different directions and perspectives, plus it has co-op so you and your lady friend can play together. That's $10.

You looking for co-op? Calling All Cars is pretty fun ($15).

flOw is also pretty good and is made by the same people as flower ($10).

That makes $25.

Joe Danger is a riot, and is really easy to pick up and play for either short bursts or long gaming rampages.

I am also super fond of their PS1 RPG rereleases, but I'm not sure how girlfriend-accessible those might be.

As someone above suggested Lara Croft And The Guardian of Light is an excellent co-op experience and Pac-Man Championship Edition DX is one of the best games this generation and well worth the money, I believe it is $10.

Wow, so many great suggestions, so quickly! Thanks, duders. 

Hmmm... I may need to sponsor her for another voucher.

I'd personally recommend Scott Pilgrim vs. The World ($9.99). A great co-op beat-em-up with an old school look and feel to it. Very enjoyable. I'd also go for Super Puzzle Fighter II Turbo ($9.99). It might be adversarial as opposed to co-op but it is fun enough that you might not mind so much whether you win or lose.

shatter is a really good modernized breakout game with an awesome soundtrack and i really liked pixel junk shooter.

How come no one suggested Outland yet? That game's pretty good.

I'm gonna throw Tales from Space: About a Blob into the pile of recommendations.

Just some feedback:
 
We ended up getting PixelJunk Shooter and Tales from Space: About a Blob, and they're both great!
 
We'll definitely need some more funds to get some of these other gems, though.