I wouldn't tell the 10 year old me anything.. I would laugh at them and say that this is your future... Seriously though, I wouldn't tell them anything...
At this school I went to, we went on a solo hike for a night, just by our self, in the bush up in the mountains, and we had to write a letter to ourselves, seal it, give it to the school, and they posted it back to us 1 year from that day. I can't remember exactly what I said, but it was kinda half arsed, wish I wrote more. This is kinda the opposite to what the topic is about, but still.