External monitor, any monitor would do as long as you're happy with it. See the hundreds of other threads about which one is best for your budget.
A Laptop stand, I recommend the rain mStand, very solid and simple.
An External keyboard and mouse/trackpad given that you pick up a screen and stand. Always nice to have though, little bit more freedom and/or comfort depending on the model.
A neoprene sleeve, I can't recommend any specific brand as they pretty much all do the same thing. Avoid sleeves with zippers, they're unnecessary with the proper design and without one there is less mechanics to go wrong.
Laptop bag for travelling and storing all that stuff, Targus makes some ok stuff, far from the best but very widely stocked.
Edit:
Some more stuff I thought about
External Harddrive 2TB (I have the Western Digital Ultra, it's great)
Headphones Any will work, but clearly some are better than others.
Hard Shell case for rMBP (I have the Speck SeeThru crystal finish)