Well, it is getting close to time to start reviewing for the AP Computer Science A exam.

What main topics are heavy hitters to review?
For sure, review classes, arrays, arraylist, and matrices.
Probably not a bad idea to do a nice review over recursion and sorting and searching.
Obviously, there is more to review, but these are main topics that are musts.

How to review?
Multiple choice review is critical. A randomized multiple choice system like Moodle or Canvas really helps. Load in lots of questions and randomly pull a few out at a time. The main thing with multiple choice is that students need to work through lots of MC questions.
For Free Response Review, have students write out old FRQs on paper and then type them up and run them. This process works great and will enable students to see where they are off on their code on paper. Plus, some of the FRQ topics are fairly interesting. The main thing is to crank out lots of code during your review.

Check out A+ Computer Science for great review materials.

Best of Luck