Just about all of it, honestly. I'm generally quite lucky if I even get a food or bathroom break. Thankfully I don't work solid 8-hour shifts and have a fairly flexible schedule to begin with, but if I'm at work I'm generally working.
My job comes in two parts: Gym membership sales/desk duties, and actual personal training. Some days I do one, some days the other, and some days both. Gym Staff days consist of at least one hour of cleaning the place, and after that it's a combination of stocking supplies, inspecting the equipment, managing customer questions/complaints, various marketing tasks, and making dozens of phone calls trying to either get new members in the door or entice former members to come back. These days are the least "fun" part of my job, but they're effective insofar as it helps me pick up new clients, which is where I make the most of my money. I don't get paid a lot for the Staff duties; the point is basically to maintain a "presence" at the gym and establish contacts with the members, which in turn leads to obtaining new training clients. The majority of what I earn is based on commission, either from membership sales or PT sales. So working at the gym is kind of a necessary evil in order to meet more people and earn more commissions. Other trainers who didn't also do regular Staff stuff inevitably picked up far fewer clients and earned a lot less money. Which kind of makes sense; If I signed your membership, by the time we sit down a couple days later for your physical evaluation and I pitch you a PT package, you're going to trust me a little bit more than if it were some other random person sitting next to you asking you to drop a car payment every month on your health and wellness.
If I take breaks at all it's generally on one of the Staff days, but it's very hard to get any time to not be "On." For one thing, like clockwork, I can make twenty calls in a row and get nothing but answering machines, but the second my ass hits the toilet seat that phone is gonna ring, and it's always someone leaving an annoyed message about how "I thought you guys were open but nobody's answering the phone!"
I also don't get to eat much, because as my boss loves to point out, "Some of these people have eating disorders so we can't have food in here, as even the smell can be 'triggering'." Which I understand, but am not entirely sympathetic towards, because A. I am not my brother's keeper, and B. Even if I'm "only" working a 5 or 6-hour shift, that's STILL a long time to be actively working without any fuel, especially if you're like me and also try to get a workout in before or after the shift. Since I never get a chance to take a real break and run across the street for something, and nobody can subsist on protein bars from the vending machine alone, I do try and sneak a quick meal most of the time, but even then I'm almost always sitting at the desk, taking quick sips of soup in between phone calls.
For another thing, it's a 24/7 gym so the place is covered in security cameras, which the boss has a remote access to on his cell phone, so taking any kind of break at all is ill-advised. Lots of times I AM working, but not doing what he specifically wants me to be doing in that moment, and I'll hear about it. Common example: I'll get a phone call from a customer about a billing issue, then sit and log into their Member Portal, play detective, checking past emails about this situation, etc., and then I'll get a text from the boss out of nowhere saying, "Quit playing on the computer and start mopping the gym."
I've been there for 10 years, so I have a little bit of room to push back, so I'm never shy about letting him know that I'm actually handling a customer and that he shouldn't make assumptions about me not working. But point being, I'm always being watched, therefore I don't have time to goof off. If I'm at work, I'm working.
I DO check in here, on FB, and skim the recent news while I'm at work, as most people do, but only in between making phone calls and only if I'm ahead of my daily task checklist. Like if I'm calling someone I've called 10 times in the past and I know for a fact they're not going to answer, I'll hop on here while the phone is ringing, since the call is gonna take about two minutes altogether including my leaving a voicemail. So I'm still working at the same time, just trying not to pass out from boredom at having to recite the exact same voicemail script for the hundredth time.
If I'm training clients I generally don't have a chance to do anything else. I don't like gaps in my schedule, so on most training days I'll schedule everyone back-to-back and will only get a break if someone calls out. And I already don't ever have enough time to work out the way I want to, so if someone no-shows, I just work out to fill that hour until the next client shows up. On those days someone else is usually using the work computer, so there's nothing else to really do except work out to fill time.
So yeah, if I'm "at work" I'm generally working, and even if I go online to "goof off" it's when I'm on hold. I'd say that my job is physically not very difficult but mentally very draining because I really don't get any "down time" whatsoever.