It sounds really stupid, but I spend a lot of my time in wheelie chairs (like, computer chairs, the ones that roll) that have the ability to spin. So, I sort of do ab twists while I'm sitting in front of the computer a lot. Like I said, totally weird and random, but if you do it for long enough, you do actually feel a little something.

I also make sure that my study space is a hike to and from my room--I use my library since my dorm is on the other side of campus from it--but just try to get walks and things in. Or, take the long way to class some days if you have time. Or, if you're a morning person like my roommate, you could get up super early and hit the gym before it interferes with any of your classes or work, and that way you don't have to worry about trying to fit it into your afternoon or evening!

I dunno if any of those are exactly what you're looking for, but those are a few of my solutions

I go to Starbucks a lot. They have a bar. If you're 5.4 to 5.8 you can stand while reading and typing without a problem. I stand for an hour in the beginning and then sit for 30. I go back and forth throughout my study session. The standing is harder when your typing then when your reading but you get use to it.

It's better then sitting even if you're not doing much. Sometimes I do baby squats throughout.

Or, if you're a morning person like my roommate, you could get up super early and hit the gym before it interferes with any of your classes or work, and that way you don't have to worry about trying to fit it into your afternoon or evening!

This is me at 5:30 every morning. At first it was a bit tough, but I am used to it and now I feel great not worrying about IF I'll get that workout in

I sit in class, doing homework, and all day at work. When going to class as well as work I deliberately park farther away than I have to so I get to walk. When going to work, I leave my phone in my car and go out & check my texts on break (which I usually don't have) but it's mostly an excuse to get up, move, and see the sunshine for a bit. I'm not very good at staying still at work either. I stand up once in a while. Or if you can manage to do it without looking like a weirdo, do isometrics at your desk.

I go to the gym every day and try to walk around as much as I can.
also recommend taking the stairs whenever possible!
I have a very small campus, but I try to go for walks in my spare time and only sit during class or when doing homework

I try my best to get up and go to the gym before my classes start for the day. And because I am short (4'11" and 3/4!) it is awkward for me to watch the tv on the elliptical, so I am usually reading.

When I can, I take the long way to class, which can be pointless since our four academic buildings are all attached and don't take much space, but I try.

I also try to be sitting in a classroom the least amount of time as possible, because I know that I am going to have to go back to my room and read, being an English major all I do is read and write. I am usually the student standing till the moment the lecture starts, and the first to be out of my seat at the end of the lecture.

Luckily, my apartment has a mini gym, so when I'm getting sick of sitting, I go run on the treadmill for 20 min. Wakes me up AND helps me with my weight.

I find concentrating on reading hard if I'm doing other things and what I'm reading doesn't usually sink in. I find night time and weekends the best to get some sort of exercise in. Even just a little stretching in the morning before I head to school.

I commute an hour both ways, so I park as far as possible from my class and walk. I also have 4 hour between class 2 days a week, so if I don't have much work to get done, I'll take an hour or more to walk around the campus or walk to the nearby village for a bite to eat, then walk back to campus.

I write it in my planner to exercise becuase if it is in my planner I will do it. Then it depends on the day on what type exercise I do. Somedays it is just walking around the block. or parking lot depending on where I am and other is in the weights at the gym. You just figure out how much time you have and then squeeze in sometime of activity.

I print out monthly calendars and schedule my workouts along with my midterms, papers, and projects. Our campus is huge so I walk around a bit more if I have time between classes. I'm an undergraduate with three kids under five who INSIST on waking up before 6am every day so that AM workout is a no go. We did manage to squeeze a treadmill into our student-family housing apartment last week, so I've been walking slowly and reading on that. It's just hard for me to concentrate on dense readings while moving.