If you have free time, definitely spend it with family or friends if you can, I know its hard to even do that during times like these. As far as hand skills go, nothing will truly prepare you from home, the best practice you can get is with the materials and instruments your school will provide you with. There is no need to worry about that this summer tho, just enjoy your time and be prepared to work hard when classes start.
There is nothing I really wish I knew, but simply a few observations i've made. I dont know what school you are attending, but most likely your first semester/year will be filled basic science classes with few dentistry related classes. This made my first semester feel more like undergrad 2.0 then dental school, but once we got into operative 1 and started working with hand pieces it was a lot more motivating and exciting for me. Also, make sure to always do what works best for you, this goes for studying, waxing, socializing etc. If your prof shows you a certain way to wax a 1st mandibular molar, but through practice you've found a way that works better for you, by all means do it. Same with studying, don't let people pressure you into study groups if you work better alone. And as far as socializing goes, dental school is an interesting mix for sure. Some people will go out all the time, and others will never go out. So don't feel pressured to go out when you need to study, and don't feel pressured to stay at home and grind when you really need to blow off some steam. Try the best you can to seek balance. Dental school is definitely a marathon, with quizzes and exams coming at you fast week after week. If you study all the time and never make room for your hobbies and friends you will burn out. So try the best you can from early on to take a balanced approach to school, while still making time to maintain your mental health. Feel free to pm me if you have other questions. Best of luck and congrats!