I love you how you think! Those are all great ideas and he would love them. I think I can do several of them with what I had in mind so far, I ordered a strip of RGB LEDs for the front Larson scanner and might as well use the whole strip around the car for effects and I’ve got an Adafruit Sound FX board I was going to put in, just need to figure out amplification, etc.
I whipped up a Buck Converter Circuit (not sure if that’s the right terminology) based on the Power Swap course to Buck the 12V down to 5V for the LEDs and 3.3V for the micro (once again, thanks @ChrisGammell!) so if those end up working I’ll be able to do lots of lights and sounds easily.
Also, sorry everyone if it seems like I’m commenting too much, Discourse told me off. 
I’m not usually a forum contributor, but there’s something about this one that I can’t get enough of.