Start by putting down a garbage bag… You’ll need a box of cornstarch… but $3 is totally worth the hour of sanity you’ll get out of it. You mix the cornstarch with barely enough water and food colouring to get it runny: think slightly-too-soft cookie dough. (i.e., LOTS of cornstarch, and probably one cup to 1.5 cups of water.)  It makes a non-Newtonian fluid (if you want to sound all sciency).  Basically, if you punch it, hit it or grab it… it’ll go solid.
As soon as you relax your hold if it (or gently push your hand into it) it’ll turn to a thick pancake-batter like consistency.  Makes cool “ooze” for little kids to play with… and cornstarch is a no-brainer for clean-up.  Enjoy.