I envy you and the surprise that awaits. You have no idea.... and that's coming from a 128 driver who has driven the 135. Make the radar install a priority...
Just relax and enjoy the drive. I drove my 128 from PA to WA and back within 2 weeks and as soon as I got home and got the car unloaded I uh, went for a drive. If you're a noob to the 135 you are going to be laughing out loud the whole trip, not to mention when your foot mashes the throttle for the first time. You'll say it's the most fun you can have with yer clothes on!
Of course if you're coming from a 335 or Z4M the feeling won't be as acute - but you'll still be impressed and you'll still have a blast.
Make sure you have the warranty info with you, especially if you just bought it. If you need service, having the paperwork handy that says you're entitled to warranty work is essential especially if you yourself are unfamiliar with its (HPFP) service history. It would suck to have a HPFP failure and have the dealer 'need to verify' the warranty. As a side note, make sure both keyfobs are current.
For me, I set my GPS to 'shortest distance' not 'shortest time'. Keeps me off the interstates and on the back roads. It's fun downshifting into 4th/3rd (in a 128 anyway) and cruising through these small towns at 45-50. Kinda gives a rest ya know?
Spend some time getting to know your seats - on a long trip I find it best to have a couple different settings so as to give some variety to your posture. If you have the seat memory, take advantage of it and set up a couple different comfortable settings. It might take a little time to dial in the right fit; when you do, with the sport seats you'll feel like you're wearing the car not driving it.
I'm old school - I packed a quart of oil, brake and PS fluid, and all my detailing supplies
Here's a video of a coast-to-coast trip.