Soft fingertips for me is the most ticklish thing I can imagine when done right. Soft, tiny little circles with the very tips of fingers and I'm squealing.
Wasn’t spontaneous, but way back in my uni days I…might have had a fun evening with a married couple. I was kinda fun back then haha. The wife briefly spent some time touching my feet and inevitably I giggled so they both spent a few seconds tickling them. Was cute.