I totally agree: Take a break and enjoy it!
I changed from my masters to doing my PhD on a weekend. The last exam was on Thursday, the first day on monday.
I regret it. I also moved to a new country during that weekend, so this was a very stressful time, with finishing the thesis, a breakup and all. So, compared to that, the first couple of days in my new job where relaxing, even if this was and still is a really hard time. But, with slightly less adrenaline powering me, I immediately became ill. Not a good first impression.
The beginning of a PhD is not a walk in the park, especially if you change topics like me. You want to be fit for this time.
In retrospect I should have taken a little holiday, even if it meant taking up a small loan. Doing my PhD I earn enough to be able to pay something back. A simple relaxing holiday does not require a lot of money. You need a place to sleep, some food and maybe a couple of beers. Try to empty your ToDo-list as much as possible, or it will haunt you in the years to come.
Using this time for preparation is probably a bad idea. You will have time to get to know your topic. If you start relaxed and undistracted (ToDo-list), then you will have a great start.