I always loved the simple fun of playing with a magnifying glass on a bright, sunny day ... you could start all kinds of small fires.
Oh ... and you know those little green plastic army soldiers? We used to play "napalm" with them. Just add a little gasoline & toss a match on them.
Fireworks! Boys love fireworks! Take that model airplane that didn't come out quite right and strap a couple of M-80s under the wings ... light the fuses ... and throw it up in the air!
Slingshots & wrist-rockets were always fun ... especially when there were plenty of birds & squirrels around.