Nice looking car!
I assume you have read the 100+ hypermile tips list above (
100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com).
Yes. Don't void warranty or piss-off your partner, who also has a stake in the vehicle. Totally good thinking. Daox is right about the grill. It can even be easily removable in design.
Hypermiling. For you, with your limited desire for mods, if it were my car, I'd:
1- slow down to 55 or 60mph on freeways and strictly speedlimit on surface roads
2- run with the tires at least 10psi over sidewall
3- shut off the engine (if it does not have autostop) at all lights longer than 10 seconds
4- find out if it has DFCO capability and use it in all situations where braking is needed
5- find out if it can coast in neutral and do so with the engine ON whenever it is safe and I want to roll as long and far as possible without the throttle.
6- keep all extra weight out of the car.
7- rarely if ever run the AC, toggling it for maximum efficiency.
8- switch to LED brakelights, if not already equipped.
9- get lightweight wheels and LRR tires.
10- get an Ultragauge.