What is your background? If you’re already well equipped for a graduate program, then I would recommend brushing up on Qualifying Exam topics and also keeping yourself from rusting. Additionally, learn programming or finance to give yourself a more employable fallback. If you’re not super prepared, work on filling the gaps on your undergraduate background so you will be more competitive for next year. I think if you keep your discipline, you’ll come out fine, albeit at the cost of one year (though you can always try to graduate PhD in four years to make up).
I’m also a big advocate of travelling for fun, but I understand that that might not be feasible for everyone. If it is for you, I’d consider spending some time doing that. It keeps your morale up, you learn a lot of non-math stuff, and you can always study while on vacation.