I Had ACL Surgery But Need To Stay in Shape ?

i'm 16 years old and tore my ACL during one of my Varsity Basketball practices. I already had my surgery and i'm on my way to recovery, but i need to stay in shape. i plan on playing in the summer and dont want to have to lose any extra weight (which i'm already starting to gain). any ideas on exercises i could do to get my arms and core stronger? any advice is appreciated(: