Prius Hubcaps
I would suggest using an aluminum pizza pan, not the steel ones they carry at Wal-Mart.
Rear Wheel Skirts
I stole these links from a post over at EcoModder.
These will never pay for themselves... considering they don't even come painted, so you would have to get that done, but you might be able to make some that are similar.

From the Tesla engineer blog years ago, "Any modification after a car is built adds weight."

Basically any after-market work you might do is going to make the car heavier and less efficient. I'm not saying you shouldn't have your fun, but you should consider the loss of efficiency in a car built for efficiency. If you check PriusChat there are some pretty wild body mods, including one where they basically made it look like a gold sports car with a rounded hood instead of that weird angular bump, a real tailfin etc.

Today I put new Michelin Primacy MXV4 tires with König Zero-In light sport alloys (see pic) on my 2007 Prius (switching out the original Goodyear Integrity at 30,200 miles).

I went for the Primacy for the handling and fuel economy. A proven tire. I was eyeing the Michelin Energy Saver A/S for a bit, but liked the Primacy handling aspect for my commute. We'll see how well the MPG holds up, but even driving from the tire company I was happy with the improved handling of the car, the feel of the road, and how much more quiet the "road noise" was.

The rims were a choice I'd been considering but didn't make the decision until they showed me them on the car, which is when I knew I had to get them. I like the look of chrome wheels, but I didn't want the added weight. So I went with the sporty looking light alloys. Interesting aspect with the wind resistance. I guess I didn't think of that.