Umm... playing with homemade explosives and experimenting with
various means of detonating them electrically.
I shan't go into detail about the ingredients that went into the
powder--suffice it to say that they were readily available "over
the counter" back then. 
There was a group of us in secondary school that engaged in these
activities. Anyway, we filled a small plastic bottle with said
powder, then fabricated an electrical detonator by soldering wires
to a small flashlight bulb, breaking the glass to expose the
filament, inserting the bulb into the neck of the bottle, then
sealing the neck with melted candle wax that was allowed to harden,
with the wires sticking out of the wax and connected to another set
of electrical wires long enough to allow detonation from a safe
distance.
We detonated a few of the smaller bottles at various remote
locations in Singapore--the blasts weren't all that spectacular,
but they were fun. The bigger ones, we blew up at a remote location
on the northern part of Pulau Ubin. Fortunately, we were never
caught.
Even at that young age, I liked things that went bang!
