In my experience the moshi keyboard cover is the one vital accessory because it keeps food, whiskers, dust, water droplets from getting down in your keyboard. The moshi fits tight too.
Since you bought a 13" I would skip all the other accessories like a mouse and keyboard UNLESS you are going to buy an external monitor for your desk space. I have the 24 inch LED display a keyboard and magic mouse at home but often miss the feel of that giant glass trackpad when using my desk setup.
Other than the moshi, you'll want a protective sleeve for your backpack in case you fall down (or off your bike) and to keep sand etc. out. Personally I think those speck cases that snap on are ****. Who cares about a small scratch every now and again. It's better to view your aluminum glory without all that plastic crap.