A major disappointment I'm afraid to say. Such a wasted opportunity. I usually love all the Scooby-Doo DTV movies, but Return to Zombie Island did not impress me. You can't help but compare it to the original Zombie Island, which is just brilliant.
This sequel isn't worthy of association in my books. I don't want to spoil anything so I won't go into too much detail, but the supernatural elements are practically discarded. Instead we get a plot that's contrived & boring, and jokes that are hit and miss (mostly miss). Also, the way Velma is written in this movie is an affront to her character. Yes she's a skeptic, but you can be a skeptic without being an insufferable know-it-all.
On the whole, the children will probably enjoy it, but for those of us who grew up with the original, Return to Zombie Island is a wasted attempt to capitalise on the nostalgia of the original movie. If, like me, you love the original Zombie Island, skip this mess.