Those are all great suggestions!

I was also thinking that maybe Honda or somebody would have a decal that would either go on the outside rear panel(s), like over the gas tank door. Or maybe a window sticker to go in either the back, or down low on the rear windows.

Or, up nearer the front wheel-well. I used to have a 2002 Honda Civic Si, and it advertised it's "i-VTEC" engine in that location. Ironically, due to confusion about the meaning of that decal and the fact that the car is sort of different-looking, a whole bunch of people asked me "is that one of those hybrids?" Nobody has asked me that about my HCH!