What an interesting topic. I'm sure there's an endless amount of sciency stuff I did (not sure if any of it was "cool" though), but I can only recall a few.
I, like most kids, had sea monkeys. Damn, those things were awasome. Unfortunately, I had no way of cleaning their little tank, so eventually they died.
I think they make all kinds of special tools to take care of them with now, including cleaning aids. Also like most kids, I had a "magic rock" kit that I loved muchly, and I always longed for a chemistry set. I tried using my uncle's old one from his childhood, but all the stuff was either too old or used up.
One of the most bizarre parts of my childhood involved "experimentations" and "observing the behavior" of insects, mostly ants. I'd do horrible things to ant hills, such as pretending to "clean their homes" with soapy water, or digging as deep as I could into their hills. Sometimes I was amused equally by doing them favors, such as putting a cheeto directly at the hole of their hill and just watching their reactions.
My favorite of these experiments was putting the bugs in water and waiting for them to give up swimming. Then I'd try to find ways to revive them... Pressing them lightly against the tv screen, as I recall, had great results.