Lincoln logs, marbles, a folding knife, any old school Tonka
For me I would also throw in my favorite....a good fishing pole
Some kid left a junker bicycle in our yard for a couple of weeks. Pops asked around, nobody recognized it. So... hauled it in the garage, and built it into a Sting Ray. Banana seat, wide slick on the rear, and ape hangers. Sprayed it green. Greased all the bearings and used fuel hose for the pedal rubber. That bike could pop a wheelie with the flick of the wrist.
Never did find out who left the bike laying in the yard.