Gap year before post-doc? I have been in a rush to accomplish academic goals since I remember myself. Started my PhD right after undergrad and did my Masters on the same time. Now I am about to submit my thesis and feel kind of burned out, having spend so many years in a far from perfect working environment. 

Academia has always been my goal (I like both teaching and research and I am not interested in industry), which is why I started a PhD on the first place, but I really feel I need a big break to think clearly of what I really want before applying for a post-doc. I also read this somewhere: "The rest of your life you are going to be a scientist. This could be your last chance to be something else. Take it."

So I thought a "late gap year" would be ideal for me. I would get to travel, which I love, volunteer in wildlife conservation and in schools of developing countries and take some time to clear my mind, so that when I am back, I can take the right decision for a post-doc and be totally dedicated in it.

Up to now I have a decent resume with a 5th paper in preparation (including first-authored) and several international conferences, fellowships and awards. However, I am very concerned on the impact such a gap would have on my CV, since I want to apply in high reputation universities/institutes, where competition is fierce. 

Should I tell a future PI I did a gap year and if not, what would be the appropriate excuse for a year off? Also would this gap have an impact in future job-seeking (mainly for positions in academia)? Finally, I am also worried about the reaction of my current PI (who has been asking me lately, which lab I am planning to apply for a PD) when I tell him my plans. The last thing I want is a reference letter from an angry PI.

*EDIT*My field is Molecular Biology. I performed my PhD research at several European countries and I am flexible with post-doc positions (Europe/Israel/US/...) depending on the projects available.
As usual, if you include information about geographic location (both the current one and the one(s) in which you intend for the future) and your field, you can get more specific advice.  Advice which is generalized across all the world's academia is at times extremely superficial.

I will speak from the perspective of American academic mathematics.  In this subculture, taking a full year off before starting a postdoc looks bad: the competition for postdocs is extremely fierce right now.  For every postdoc position there are at least five other people who wanted that position but couldn't get it.  So employers really want to give jobs to people who are sure that they want them, not those who are feeling "kind of burned out" or "need a big break to think clearly of what I really want".

If you take a full year off instead of applying for postdocs, then unless you have something amazing to show for yourself at the end of that time, the year off will definitely hurt your applications.  Whether or not you can actually do work during a "year off" seems highly field dependent: in mathematics this is certainly possible; in laboratory science this seems much less feasible.  

Here is some advice that I would offer you:

1) If you want to take some length of time off, try to secure a job upon return before you take the time off.  

If this happens then in some sense you do not really have an employment gap, and that will look much better on your CV.

2) Consider taking a smaller amount of time off than a year.  

A year is a really long time to put aside one's career.  In fact many people would have trouble supporting themselves (especially if they have families or dependents, which I guess you do not) over such a long unpaid stretch.   It is also more than enough time for your academic skills to atrophy.  In some academic fields (pure mathematics not so much, although in some subfields this could still come into play) a year off is enough to make your entire research program less fresh and cutting edge.  Anyway, imagine that you are competing with many talented young people who spent the first year after getting their PhD working their butts off.  Do you really want to spot them an entire year headstart?  As other people remarked in a closely related question: whether it is fair or not, you should imagine that a big clock in the sky starts ticking the second you receive your PhD.  From that point on, people will be evaluating your work not just in an absolute sense but relative to the time elapsed from that point.  Adding in an extra year makes almost anyone's profile look much less strong.

I think you should consider taking a shorter amount of time off: either a semester or a long summer.  As the American academic calendar runs, you will have a built-in vacation of about three months just by virtue of being an unemployed PhD over the summer.  I really enjoyed this time: I moved into an apartment downtown in the city where I grew up but hadn't spent more than a few weeks at a time for my entire adult life.  It really was refreshing and recharged my batteries.  However, it also depleted my savings: by the time the new semester rolled around, I really needed the paychecks.  

If you need much more than three months' break from a career, you should ask yourself: are you sure that this career is really for you?