Visiting family over the holidays is very important for us, and has proven to be one of the main difficulties of not having a car. Originally, when planning our car-free adventure/life, we'd calculated spending a certain amount on car rentals for trips to Connecticut to visit family--usually we can rent a car for $30-$40/day from the airport over a weekend. However, as we discovered at Thanksgiving, car rental companies will triple their daily rate over holiday weekends. So our little three day trip at Thanksgiving cost more than $350 just for rental, plus we had to pay for gas. It was worth it, but more than we'd expected (by a lot).
Over Christmas, we made another three-day trip to CT, but this time we used Zipcar. It turns out that you can rent a Zipcar for a maximum of four days. The daily rate does not change over the holidays. We rented a Honda Element for $89/day, which is a little less than what we'd have paid from Thrifty or the other big car rental agencies, but all the taxes and gas were included, so we came out about $100 ahead. And I didn't have to schlep myself over the the airport and back. Over Thanksgiving, it took me 90 minutes just to pick up the car. With Zipcar, I just needed to walk 5-10 minutes from home.
There are some cheaper car rental places around, very small companies, but the online reviews of them are pretty scary. If anyone in the Boston area knows of reputable ones, I'd be interested in hearing about them. For now, though, it looks like Zipcar will help us out over the holidays.