Great stuff! Road trips in an M3W are fun, and road trips in the US of A are amazing!
If 1-up, then choose a waterproof bag for the passenger footwell (I drive with the half tonneau on). Water bottle in bag on the passenger seat, sat-nav or phone on the RAM mount under the tonneau to be shaded. Suitable headware and goggles of course (agree, helmet for freeways - insect or stone kicked up by a tire would not be nice). Easy access to waterproofs and if touring in Summer appropriate sun protection. Carry at least 1 inner tube, maybe AAA etc list of local tire shops to help with any fixes (for long distances I might be tempted to buy a new wheel+tire and put it on the trunk rack like classic M3Ws!). Optionally add a GoPro on the headrest as a "dashcam" to record more interesting experiences, but note when you get out it adds to the list of thinks to secure before leaving the car.
Most important, have fun and post photos here
