Neil Patrick Harris is fast becoming the busiest man in show business, so it's fitting that he'd be working on Labor Day. The How I Met Your Mother star — whose summer activities included hosting the Tony Awards, starring in the box office hit The Smurfs, and a guest-judging stint on So You Think You Can Dance — guest stars on next Monday's episode of the Cartoon Network whimsical series Adventure Time (Sept. 5 at 8/7c). The gig came about when show creator Pendleton Ward approached Harris via Twitter last December.
Titled "Adventure Time with Fionna and Cake," the episode is, according to the network, set in "a gender-bending, parallel universe." Fionna and her cat Cake are the female versions of the show's main characters, the fearless boy Finn and his magical dog Jake. Harris plays Prince Gumball, the object of Fionna's affection. In this exclusive clip, the prince serenades his lovestruck companion.