Then they can spend the session striding like colossuses through the Realm of Dungeons & Dragons (maybe they need to find a pretty flower to defeat the beholder with) before being shunted back through the portal to their workaday perilous adventuring lives. It might be a kind of April Fool's joke--just not a very good one.
This idea comes from the Star Trek: The Next Generation episode ("Remember Me"--4.5) where Dr. Crusher gets trapped in a warp-bubble universe, or any of the other stories in the world where you spend some time in another world--like when Jack Tripper was going to win the lottery and, as a kid, I was very worried that that would destroy the show and I wouldn't get to watch it anymore (though I guess that's more the threat of an alternate dimension, and only sort of). Did you ever worry, when you watched TV as a kid, that if the world was destroyed in the show you yourself would be killed? That was the best.
It's funny to me to play Dungeons & Dragons--the animated series--as a Cartoon Action Hour series. You should do it too.
Oh jeez, don't forget the bleating unicorn foal!