I know Windows Store apps aren't the best, but if you have a couple of things you want to buy 20% is not too bad.
I'm hoping to get the Drawboard PDF app that cost $12 so I'll be getting some use of it (However I have no idea how to spend the rest of the credit!)
Available on their website is $50, $25, and $10 gift card.
Isn't the same balance used for XBox games and stuff too? Not too bad in that case.