Depends if you want it to be pleasant or super gross - anything sloppy is a good bet - custard, pureed or mashed fruit, rice pudding - or if you want something that's icky - try beans, tomato soup, ketchup or curry sauce.
Thanks for your replies and to those who private messaged me. They all sound fun to use, and the icky ones, ie. ketchup, curry sauce....yes am up for that too Ok....going to be alot of hair washing this week I think!