Calisthenics is the way to go. You can start without any equipment and you can do it indoors and outdoors. There are plenty of calisthenics tutorials on youtube. Apart from that I suggest making a 20-30 minutes walk around lunch break
yes get some olympic rings. look into the (free) app You are your own gym(Yayog)
Buy overcoming gravity (or join the reddit group where the author answers many of the questions). Working out with bodyweight is fantastic imo
