Speaking to EW, director Gary Rydstrom says that Pixar got back "everybody" back for the short—that is, all the main cast members of Toy Story 3. And he sums up the film's plot, which was leaked last month:
"Ken wants to take Barbie on a perfect vacation to Hawaii by stowing away in Bonnie’s backpack, but they get left behind in the middle of winter while she goes off without them... All the toys see how devastated Ken is."
That 'motivates' the toys to recreate Hawaii right in Bonnie's room.
Rydstrom also shares his favourite moment from the short, which I suspect will be my favourite moment as well: Chuckles sings!
Hawaiian Vacation will be in theatres with Cars 2 on June 24.