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If you block off part of your grille, be sure to monitor your engine temperature using more than just the indicator on your dash.
Regarding wheels, switching from 18lb OEM alloy wheels and 17 lb tires to 11 lb HX rims and 16 lb tires on my Civic gained me 2-3 MPG. Lighter wheels find their biggest benefit in hilly driving and city driving.
My rims were 15" and downsizing to 14" required using a 1/4" spacer. May or may not be a problem for you.
Since you also live in Michigan, I would recommend getting winter tires. From what I've heard and seen Bridgestones are the best summer tires and Nokians are the best winter tires. Michelins are right up behind them.
So to avoid having to remount tires every time the season changes, I would pick up a set of lightweight wheels for summertime, put summer LRR tires on them, and then put winter tires on your steelies.
Going with a smaller diameter tire will essentially shorten your gearing and reduce fuel economy in most situations except hard city driving due to the smaller rolling diameter. Increasing your diameter will improve highway fuel economy, but slightly reduce city fuel economy due to the heavier weight. Going narrower is a good alternative.
Both Insight rims (10.5 lbs) and HX rims (11 lbs) are excellent choices. If your stock tire size is 175/65R15, you would need 175/70R14s to keep the same size. Going narrower would be 165/75R14s, both of which aren't that common to find.
Best bet would be Insight wheels + Bridgestone Potenza RE92s and a taller transmission swap to offset the shorter gearing from the wheels, but the return on investment for that would be in the hundreds of thousands of miles.
Don't spend a lot of money on rims if your goal is to save money. The return on investment will never be there. I spent only $100 on my HX rims and they haven't paid for themselves...yet. I will be using them as wheels for my Insight's winter tires, and that will count as a break even point for me. Insight rims are generally harder to find and more expensive than HX rims.
Smooth coroplast discs are a very inexpensive way to do smooth wheel covers, but your opinion may vary depending on how important aesthetics are to you.