foam is probably the easiest, yep. and you can do a lot with foam to make it look different. makes the clean up a little different but not by much.
this does not mean foam doesnt have its "stuff" it dries out the hair really badly, i have to walk around with conditioner in my hair for days and it breaks my hair off at the ends (but i have a lot of spare hair) haah and it dries out the skin but its fun as hell to play in! you can color it. add water and make it a slime, there are tons of things you can add to it to make it into a totally different type of moving substance!
Vanilla pudding and whipped cream. My wife and I love it. There can be a bit of an odor issue following the mess, but we have found that a good, HOT shower with plenty of soap helps to avoid odors.