Nice Paseo... How many miles are on it? Auto/manual?
I would suggest getting instrumentation in there so you can get realtime monitoring of your MPG. The MPGuino wires up easily, I have mine where the factory clock was, it's a nice spot for it.
If your commute is mostly highway, then the extra weight from whatever audio stuff you put in won't make too much difference, once you're moving, you're mostly fighting aerodynamic forces, not friction. There is a larger electrical load though, which means your alternator will be putting more strain on the engine.
For aero mods, the basics are grille block, belly pan, wheel covers, and wheel skirts. Grille block is easy, just have to make sure you don't end up running the fan too much. Actually, depending on where your license plate was, removing it might actually make it less aerodynamic.
Different gen Paseo, but you should still check out Daox's
Paseo build thread if you haven't already
Why nitrogen in the tires?