I love the package 6, BC. Its got to be the best package. I use every feature they put in this package often. The voice activation is a little bit trivial but I say I'm Hungry and POI off the most with that.
The heatshield sun shade is a good idea...it is very hard to find a sun shade that fits and they make a nice, good one specifically for Prius. You can get a discount and support PriusChat--use this thread:
Extremely popular is adding the integrated XM Radio unit that works via your MFD.
There are several sources for this...a search here will find you links. Prices and install instructions vary, but the product itself is identical at all 3 main sources.
There is a sun shade available for the MFD...I've had one since they came out and if you really want it it's OK, but I don't find that it's all that helpful for the worst glare situations...that very low angle light coming in through a side/rear window.
I have a wheelskins.com leather steeringwheel cover that I particularly love.
Many of us have the mud/splash guards. Several sources for those as well...~$60 and easy to install. They really help keep some of the spray off the higher parts of the car.
Window tint looks great and can keep the car cool.
Things like the EV mod from Coastal tech (or do-it-yourself) are popular and fun, but not necessary or particularly benefitial.
Taking look at the Coastaletech.com site is a good idea as something like the auto door lock may perfectly suit your lifestyle, but it isn't for everyone.
Some of us have gotten the rear bumper protector made for the Toyota Matrix or some other style more durable than that made specifically for the Prius.