I would say Kettlebells. I would say some kettlebells exercises like swings would compliment your Boxing training while being different. They will also take up very little space in your home in terms of storage and working out. Also, walking is outside and dependent on the weather but I would really recommend walking daily to counteract sitting.
I found kettlebells to be the most time and space optimal exercise, the only downside is that you really have to get someone to show you how to do it properly, but after this you can burn 300-400 kcals in a 20 minute workout.
A friend of mine is a big fan of him https://www.amazon.com/Kettlebell-Simple-Sinister-Revised-Up...