I also have a $50 gift card. Problem is, everything at Best Buy is cheaper on Amazon. I have Prime, so Amazon is much more convenient too.
I think I'll probably buy the Magic Mouse at BB. It would be cheaper on Amazon, but I want to use the gift card before I lose it.
I just bought a neoprene sleeve for transporting my MBP and its nice. Its made by Incase and has a faux fur lining so it doesnt get scratched up. It ran me $40. Other than that I don't use anything else with my MBP.
IMO, any Macbook Pro user can get great mileage with a laptop pad. These things get uncomfortably warm with extended use on your lap. Best Buy actually has a very good selection to choose from all within that price range.
It may not be as exciting as some other suggestions but a second power supply is a good option ($79). I keep one power supply routed up through my normal desk setup and keep another in a backpack for travel or use in another room when needed. It's actually a great convenience item.
I am going to be getting a $50 gift card as well (credit card rewards). I plan on buying a magic mouse for my new 13". I figure I'll get the magic mouse because chances are I won't be able to find that at much of a discount elsewhere and I like to use a mouse when I am at my desk. So yeah, my vote is a magic mouse.