Don't study anything. You'll hear that from everyone. It's a waste of time...time you'll regret you wasted instead of just enjoying your time off. Any basic science you study will probably only benefit you for a week or less in school. Maybe only hours. And while content might be familiar, it can be presented differently and in varying depths of detail.
You can't (shouldn't) do anything for boards. Especially if your school takes it after the second year. No matter what school or level of preparation you're at, you're probably going to end up studying decks/First Aid/old exams for two weeks before you take it. Now that part I is P/F, they're not a big deal. Not something that requires that much preparation anyway. Much less than the DAT. So wait on that.
Be ready to study, especially if you've had some time off from school. What you did in undergrad might not work in dental school. There's some med student on youtube that gives some tips on studying and I'm sure there are lots of other resources (like SDN). Look into them so you have some methods to fall back on after you pee yourself after you fail your first exam. There's not time to cry; you adapt and keep chugging. You'll hear this too: school isn't that hard if you try - it's maintaing top rank that is hard. Just be ready to work.
Random thought...if you don't have an exercise routine, perhaps try to start that habit now. There's time for it. There's time for lots of stuff, actually. I think staying active can really help in many ways. If it's a habit, it'll be easier to make it happen instead of couch time.
The only thing I've suggested to those who absolutely can't sleep at night thinking they should be studying something, is dental anatomy. And this is only if you've had very little exposure to dentistry. It can be a different language and it might help a little in lecture and when jargon starts flying quickly in lab. Nothing too intense, just start to learn some terms and learn to appreciate how much is involved in each and every tooth. It's kind of crazy.
As for hand skills or waxing....forget about it.