In adult life, being already curious about pee:
My girlfriend needed to go after being on a local carnival on the way home walking. She told me, I just need to go in this playground sand box, pulled down her pants and let it run for a minute. I got erect immediately and got hooked for life.
Earlier, also in a sand box, I had nice expierences as well... but as I have been only 5-6 years, this is not something to post here at all.