So I have an adjustable shower head that you can take off the hook and hold. Well, I like to put it in jet mode, sit on the ground, and aim the head right at the soles of my feet! It tickles so much and I love it!
I remember reading the classic 'Alice in Tickle Land' story when the Queen aims thin water jets at Alice's feet, which drives her crazy! It seems to be one of the few devices someone can use to tickle themselves!