I did it for the office at Halloween. Since there was going to be so much candy on it, I used a box mix in a 9 x 13 pan, making a wretched face out of Twizler mouth and eyebrows, Lindt white chocolate truffle eyeballs, a Milky Way nose, and Bubblicious gum tape tongue.
The candy coating was chocolate chips (with some leftover white chocolate chips), mini Reeses cups, M&Ms, broken-up Hershey bar with almonds, mini non-pareils, and some chocolate-covered almonds. Do use the chocolate ganache for the frosting; aside from being melt-in-your-mouth good, it is sturdy enough to hold the candy. Also, when you choose candy, think about how they will go together. I almost got Junior Mints, then realized that mint might not go too well with peanut butter and almonds.