My job currently has me sitting idle for hours each day with little more than my work phone and a pen and paper. What would be the best way to spend this time?

Read a book (if you can?)
Write 10 ideas per day for how you can do something you’re passionate about. Pick a new subject each day. If you like computers, for exmaple, 10 ideas of things you want to learn how to do. If you like dogs, 10 ideas of how you can spend more time with dogs.
Write a book — fiction, non-fiction, romance, comedy, drama, self-help, doesn’t matter. Doesn’t even have to be a good book. Just write it
Write your life story, starting as far back as you can remeber. Time will fly by and you’ll remeber things you’d long forgotten
Write handwritten letters to family and friends. Not for a special occasion — just to tell them you’re thinking of them and that you appreciate them. In this digitized world, a hand written anything will blow people’s minds.
Write down every life question that you want to know the answer to, then post them all on Quora later
Write ten ideas for how your job could better situate your work setup so you’d be more stimulated
Write out a detailed vision for the life you want to be living five years from now
Make a list of all your personal accomplishments, big and small
Write a list of everything you want to do and have in your life by a certain time (5yrs, 10yrs etc) and then strategize how you’ll do it