Enterprise in my town often has a weekend $10/day low-mileage rental special. (80 or 90 miles a day, I think?) Could be worth it to see if your area has something similar.
For gas, are you using gasbuddy.com or a similar site to make sure you're filling up at the cheapest gas stations? Not worth going way out of your way, of course, but often going just a few blocks over, 1 highway exit farther, etc. can make a big difference. Some gas stations also have a program where you can set up direct-debit to your bank and get a 10 cents/gallon discount (you give up credit card rewards, though, so be sure to do the math that way). Cumberland Farms is the one near me that has this program, but I imagine gas stations in other areas may also have it.