Historically, I've used a deck of cards - choosing a picture card or an Ace means I spin the wheel to choose what substance I use (gunge, eggs, syrup etc). I use a fantastic app simply called Spin the Wheel - Random Picker.
I then spin a second wheel to decide where the mess goes (head, feet, down the t-shirt etc).
If I pick a Joker, I get to dump a trifle down the boxers.
It's a fun game, although always open to ideas....?
I'm looking for something maybe with an bit more 'peril' or something that I have less control over. It's difficult when you're alone but I'm sure you're a creative lot....