i want to get a 5!!!
Wondering the exact same thing
Stoichiometry of course, acids/bases, Gas laws, thermodynamics
For free: Google AP Chemistry summer assignment, lots of high schools have them online.
If you have a budget: buy the book Calculations in Chemistry by Dahl and Nelson and work through it. (If you’re on AP track math, you will be able to skip all or most of the math chapters and just do the chemistry stuff. In that case, it’s not unrealistic to do the whole book. If you need the math, you won’t finish it over the summer, but plug through what you can.)