As far as putting the items in a box, I wish I had a place to do that. We don't have a garage and we don't have a porch; just a small concrete step by the front door and I wouldn't really want to have a box eternally sitting there.
I like the idea of putting the items by the curb or some other visible spot like the electrical box a poster has mentioned.
Wait 5 years, and see how you like it when the young couple down the street is too unfriendly to bother yelling out the door (is this your son's hat?) It's neighbor thing, are you really that busy?
The thing is, I find this stuff when no one is around and we hardly know the neighbors other than the 2 right next to us. It hard to catch the kids out there, or parents outside to yell down to. As far as being busy, yes, I am. I work full time from home while also taking care of my son... not a whole lot of time in between to be picking up for neighbor kids who I don't even know. When my husband comes home we're busy with dinner and sometimes errands.
Sure, if I happen to see my next door neighbor outside and I have a suspicion something belongs to their kids, I will definitely ask; more often than not though it's hard to just run into them by chance and I'm not going to go around knocking on their doors all the time.
In the future with my own son I think it will be easier; I'm sure I'll actually know the kids and would be more easily inclined to pick up the phone and let them know they left some stuff behind. But I don't know any of these kids... sometimes I find orange cones set up in our back yard left over from when they were playing football. Guess who always has to go around and pick them up when it's time to mow? There have been some times footballs come flying up and slam into our sliding glass door and land on our deck. We've had kids shooting paintballs at our white shed... After a while all this stuff adds up and really makes me not want to go out of my way to pick up after their stuff.
Sorry if I'm being cynical, and maybe my view will change in the future, but right now it just kind of bugs me since it's an ongoing thing.