It really depends on your curriculum. We didn't get the first summer off - we had 2 weeks between august and september and were encouraged to take the NBDE part I at this time which I did. Even though we had a summer semester, we had covered pretty much everything that was on part I outside of some pathology during the first two semesters.
I would take the board exam if you can. Again, I have no idea how your curriculum is set-up. I mean, if you haven't taken micro or biochem yet, you should hold off - but it seems most schools get these out of the way during year 1.
If you can't take the exam, then I would seriously just do something not related to dentistry. Research is great if you can get a meaningful position and are truly interested. However, if you are merely looking to 'pad' your resume, you will have a miserable experience. Not to mention you won't get much done in two months (unless you already have experience) and will have very little time to continue once school starts again.
But on the other hand, it is something to try out if the opportunity arises and you have an open mind to it.
D-2 year is the hardest year for many students so I would either lighten the load by notching off the NBDE, or simply get out and enjoy life.
You are going to get tons of different opinions, and really - the best should be your own. Every school is different, everyone has different goals - there is no correct way to spend your off-time.
Regardless, I would focus on surviving your next semester before you really start thinking about your break. Good luck.