Simon,
Using floaties to try to walk on water - that is hilarious!
The go-kart experience reminded me of when I went to a friend's birthday party when I was about 7 or so. His parents had got him a little mini dirt bike (motorized). I got on it and started riding it, but nobody had told me how to stop it, and I ended up crashing it, much to my friend's dismay. His parents just laughed, they thought I was a funny kid.
Your swimming pool experience reminded me of how I got a Speedo swim suit (the kind that looks like ladies panties instead of shorts) when I was 12. I was a skinny skinny skinny kid who had no business wearing a Speedo. People just stared, I think they felt sorry for me! "Don't his parents feed him?" was what they must have wondered to themselves.
But so far I think Dolphin57 is the geekiest. The bedroom DJ, too much! That reminded me of how I made a fake guitar for myself and would play along with the MTV.
Anybody else?