If you can get a minivan, that's your best bet. As already stated, they're typically cheaper than SUVs with more cargo space. You'll get a Grand Caravan, Sedona, or Sienna, each of which has fold-flat third row seats. A month ago, I moved into my current place in one trip with a rented Sedona. You can fit a lot in most of them. I would also suggest joining the rewards program at whatever place you choose. Check with your car insurance company for rental deals, as some have partnerships with certain companies. My insurance company (USAA), for example, waives several fees, gives unlimited mileage, and provides a significant percentage discount at 4 rental car companies. Look at every chain in the area to see who is cheapest.
Having rented at Hertz the entire summer for my job, I can tell you a few things about their policies. Not all companies are the same and I encourage you to do additional research. For Hertz:
-Locations away from airports are almost always cheaper. We're talking 10-20% in many cases, if not more.
-Rewards members get access to a coupon section on the website that normal people don't get. Often, there are coupons for weekly and weekend rentals.
-A lot of things are unlimited mileage, but not all rentals. Double-check your rental agreement to make sure.
-Hertz has no location restrictions in the lower 48 and Canada, at least not that I've seen. You can go pretty much anywhere in the two countries unless told otherwise.