Need help: working out and getting strong

Hey so I wanna start getting strong again, I was pretty fit for some time but I stopped working out and I lost my muscle mass, and I wonder if you had tips on ways to go back to that but without doing repetitive exercises and such. I used to boxing and that was pretty neat, I just can't do with how monotonous gyms are and I need something consistent but entertaining. I'm thinking of getting back into boxing soon but I'm moving so that's a no for now, if you guys had any tips or ideas as to what activities I could get into, let me know :thumbsup:

I'm mainly interested on working on my back and shoulders cause my posture isn't terrible but I crouch when I sit and it hurts a ton. Also wanna gain some muscle on my arms and tone most of my body. Thank you :)