Wow, this is my exact situation! I am also starting grad school this Fall and have been our of school since 2015 working full time. I have this month off and have been re-teaching myself some areas that I've been rusty in. But I agree with the other posters, it sounds like you're doing everything already...and it's important we also enjoy what time we have before things get hectic :).
A good suggestion was if you know the kind of research you will be doing, prepping (even very preliminary) REB, or if you are applying for funding/scholarships then working on your ccv (if applicable).
The other suggestion about ref softeware is great actually. I should probably do that...
Also if you are lucky to know ahead of time if a course requires specific textbooks, maybe now is a good time to look into purchasing.
Above all else, try to enjoy the time off!