Best Accessory - Tire Pressure Gauge
Cobradav gives good advice. (And the red leather interior is stunning, CD.)
Get a really good air pressure gauge (and an inflation pump). When you go to the dealer to take your baby home, the odds are high the dealer will have forgotten to reduce the shipping tire pressures. Mine were all around 45 ppi. The front is supposed to be 32 ppi and the rears on the convertible 38 ppi (unless you carry heavy loads or go over 100; the book tells what the pressures are then).
Reduce the pressures to say 33 / 39 and have the BMW people show you how to "reinitialize" the tire pressure system ("FTM" which I suppose stands for "flat tire monitor").
The FTM works by determining the rolling radii of the tires on each axle, using sensors already in place for the ABS system. If the sensor determines that one of the tires is rolling straight ahead at a different rate than the other on the same axle, and the difference is sufficiently large (there is, of course, an algorithm for this but don't ask me, I'm a lawyer), your FTM will go off because a tire with less air will roll at a different rate from a tire with air. So, whenever you change tire pressures or change tires, you must reinitialize the system so that it "matches" the tires on each axle. (When I tried to do it all of my iDrive problems started, so I think it is best to do it at the dealership.)
After you have "reinitialized" the FTM (a dialogue box comes on the iDrive screen saying FTM is "active"), you are set. The system now has a baseline reading for the tire pairs on each axle. If you ever get the FTM warning, the tire pressure gauge will be invaluable to figure out which tire it is.
This was one of the most important features for me on the 650i. After I got my CL55 4 years ago I had 2 explosive blowouts within 3 months. (The OEM tires Mercedes places on its cars are junk. I finally got rid of them and put on Michelins.) Mercedes did not offer a tire pressure monitoring system in 2002, only in 2003 and later. And, the Mercedes system measures and reports actual tire pressure in each tire, rather than differences, and is prone not to work. The BMW system is supposed to be bullet proof. But, since it's an iDrive function, you never know!
After you've lived with your car for a while you can figure out what else you will need.
Please tell us all about your experience when you get your car, and as you break it in. Can you give us the vitals - colors, options, etc.? I hope you have a 650i with absolutely no iDrive issues. If so, there are lots of guys on this Board who will want to clone your software! (So, also tell us your build date, please. Chuck92103 is keeping track of this stuff for the class action lawsuit.)