I enjoy swimming, so much so that at work I have dug a trench between my desk and the kitchenette and filled it with water. This allows me to keep up with my swimming everytime I make a cup of tea. To bring the tea back to my desk I place it on a float and nudge it with my forehead.
I’m not a total moron though as I have built a series of bridges over my moat so you don’t always have to swim. These come in handy when I have visitors who may not have remembered their goggles.
Supermans (If you sit at a desk a lot, you should do back exercises like this)
Squats (no need weights, squeeze cheeks as you go up if you want to include your bum)
For Abs I literally do this routine: https://www.youtube.com/watch?v=8E4e2njNO0A and so far I’ve gained significant ab muscles (I take a break in between though). It’s important to do the Supermans along with them though, to keep a balance between ab and back muscles.
None of this requires any equipment, just a blanket on the floor and enough space. Aim for 3 sets each and do repetitions depending on what you can do, don’t destroy yourself and run around with muscle soreness, but also don’t take the piss and make it too easy on yourself. I’d say take about 30 mins for this either daily or every 2nd day, if you reduce time and effort, results will obviously reduce as well. Workout in the morning before eating is hard at first, but your body gets used to it. It also helps with sleep at night apparently if you workout in the mornings (well, unless you almost knock yourself out and fall into bed super exhausted and half dead ^^).
Edit: Make sure you do the exercises correctly. I.e. watch a video about pushups, most people do them very wrong. Also work both directions for any exercise, i.e. when squatting go both down and up at a slowish pace, don’t like shoot up with force and then let gravity do the work on the way down. Rather do it right and do less repetitions, no one cares if you can do 30 fake pushups and it will not help your fitness either