Off season training is a very important part to player development. If you want to become an elite player you need to play Basketball however other sports do help you become better at Basketball. Soccer will help your foot speed and foot work, while volleyball (especially beach volleyball) will help your jumping ability. 

Canada's youth Basketball off season is the USA AAU season. If your a player who would like to become an elite player the best way to get an NCAA scholarship is to play AAU or play in NCAA school camps. 

While in the off season the best way to improve is to workout.