I am writing this on my officially approved break.
The short answer. You are not allowed to do your own things while on your employers time. If you dont have so much free time, speak to your manager about getting you more responsibilities, along with a salary increase that comes along with the increased responsibility.
Anything else, is outright theft on your part, unless they sanction downtime and let you learn, eat, browse the web, shmooze on the phone etc…