The battery meter is self-calibrating, so even a weak battery with only 30% of its original capacity can still be "filled".
Get the right tires and, as Cowmeat suggests, stick with the stock rims. They're absurdly light and probably the most aero rims to come stock on any car.
Biggest aero returns will be a grille block. With the stock thermostat it will want to sit at ~184F, but the car runs really well if you can block it off such that it floats between maybe 190 and 205. Any higher than that and you're wasting energy running the radiator fan.
Biggest returns otherwise will be with a kill switch and a system called "MIMA", which allows for manual control over IMA behavior. However there's a thread over on InsightCentral, where a member (mudder) is designing a lithium replacement system which can be charged at home and has MIMA functions built in. I'm following his progress closely. Chances are good it'll hit the market within 6 months and allow for hours of constantly low assist, which should help to deliver astronomical fuel economy numbers.
EDIT: Check out my build thread in my signature for lots of little tricks, tips and fixes. Next on my list is to finish installing my outside air temp fooler, and to remove the rear washer fluid pump, tubes and nozzle on the roof, in addition to the antenna mount.