Probably a Burton Nug. Anything that's stiff enough for your weight about 10-15 cm shorter than you normally ride would be the equivalent of a play boat vs. a longer (9-10 ft+) kayak.
Though I've been on both kayaks, and while the play boats are fun, stable they are not. You're all over the place in those things. I'm willing to bet a short board would be similar. Fun to just tootle around and pop off things but I wouldn't try doing any real carving on it or big air...