Not entirely free, but may already be readily available where you are: Water cooler jugs. The standard US size is 5 gallons, which comes out to a little short of 20kg/44lb when full. At the simplest level, take every opportunity to be the one to change the jug in the cooler instead of pretending not to see that it's empty until someone else comes along.
If the situation allows it (i.e., this may not be the best idea if the water cooler is the one at your very formal office!) you can do a lot more -- hug a water jug and do squats. Or squat and hold (30 seconds? 90 seconds? to failure? totally up to you...).
Walk your water jugs up and down the room -- holding and carrying heavy things is great training and very different from lifting while standing still. Try holding a jug out in front of you instead of hugging it close to your body; the arm and shoulder work is very noticeable. If you get the water jugs with handles, try farmer's walks with one in each hand.
Not quite so recommended: throwing exercises. Unless it's a very hot day and you're outdoors and willing to sacrifice a few bottle deposits. Just saying.
Bonus: hydration is readily at hand after the workout.
no subject
If the situation allows it (i.e., this may not be the best idea if the water cooler is the one at your very formal office!) you can do a lot more -- hug a water jug and do squats. Or squat and hold (30 seconds? 90 seconds? to failure? totally up to you...).
Walk your water jugs up and down the room -- holding and carrying heavy things is great training and very different from lifting while standing still. Try holding a jug out in front of you instead of hugging it close to your body; the arm and shoulder work is very noticeable. If you get the water jugs with handles, try farmer's walks with one in each hand.
Not quite so recommended: throwing exercises. Unless it's a very hot day and you're outdoors and willing to sacrifice a few bottle deposits. Just saying.
Bonus: hydration is readily at hand after the workout.