1. Do Not Look For Your Next Job: You know internally if you're at the point in your career that taking the summer off is realistic. Don't start looking for your next job right away. You'll find it (or it will find you), and it will fill your mind space, or even shorten your planned sabbatical (then you'll need another one!).
2. Break Your Old Routine: Three days and a plane ride after quitting my job I was crashing out on a friends couch across the country (thanks Lori!). Shake your life up enough that you don't realize you've left your job. You'll forget about your BlackBerry in two days.
3. Leave Your Identity Behind: If you've ever moved cities, you've said "Back home at originalCity....". It takes time to adjust to big changes, so correct yourself if you refer to your old team/workplace/etc in present tense. Many people and social circles define will assess your value based solely on your job; you'll need to feel comfortable telling people you're taking a break. It took me a while to really let go of this.
4. Travel: Find something interesting going on in a different city and go there. One friend suggested I go tour around Europe for a few months; if you can do it, go for it.
5. Turn Off Your Alarm Clock: Unless you already have a morning ritual, turn it off. Wake up when you feel like it. As new-agey as this sounds, give yourself permission to do nothing. All those "if I had time, I would do it" things can wait (see below).
6. Don't Code: Or if you do, don't code in your normal language (admit it: you really want to learn ActionScript :-). Your skills will still be around in two months.
7. Turn On Your Alarm Clock: So, it's been a few weeks of laziness, make yourself a daily ritual. Set your alarm clock again. It's okay if you were up late the night before, just take a nap. Now is the time to make and start working through that list you've been ignoring.
8. Resist Gossip: I still hear from friends at my old job; nothing has changed. Each time, I I feel stress returning in my back muscles. When we hang out now, I try to guide the conversation away from all the usual rants. "You sound like you could use some time off to relax."
9. Hobby: Pick up a hobby or two to fill your time. Unlikely retirement, these can be hobbies you have no intention of keeping after the summer, so go out on a limb and try something really different.
I still have six more weeks of planned vacation left before I start really getting into job hunting mentality. Time will tell how it ends.