I agree basketball has incredibly useful spillover for soccer - especially in learning how to get open on throw-ins and playing/moving/defending off the ball I was initially skeptical about indoor soccer, but at 8 and 9, it can be useful if played with other skilled players with active refs. Hockey is an apt description when it is not supervised. But futsal is the closer analogue when it is supervised and played between among more skilled 8 and 9 year olds. We played in a travel league and it definitely brought our team closer together when other teams have winter breaks. It also toughened some of the kids up. Connecting play significantly improved over the winter. Beyond 9, I can imagine that the field would be a little too crowded and the activity a little too rough. Fairfax Sportsplex winter leagues are good and the refing is solid.