We are travelling (also without our kids!) in September, and after crunching the numbers, we are renting a car for one day. We arrive at 10 pm, so I'll go food shopping that night, souvie shopping the next day (Belz), and return the car at the S/D within 24 hours. This is costing me a total of $34, and with the car return at S/D we can just hoof it (or cab) back to BWV pretty easily.
Going back to the airport, picking up a car at S/D pushes the price around $60, so we'll just catch a cab if I can't get that price lower.
This way: $34 from airport, $40+ to airport = $75 - 80.
Rental car: $130 lowest I could find so far.
Towncar: $90 plus tip = $100.
Less convenient than the more expensive options, maybe, but without my 3 kids in tow, I don't mind a little hassle.
Just a thought for you.