Oh, oh, I know...a table!
(OK, y'all, back off, it's an inside joke....)
Seriously, it's a good question with some good answers.
Birdhouses, treasure boxes, shelves, bug/reptile cages, robots,
toolbox, child-sized workbench (sawhorses, etc.), hiking sticks, toy
boats, paint easel.
Safe processes they tend to like: sanding, gluing
Techniques to introduce gradually: measuring, cutting, design
Advanced: turning, scroll saw
Attitude: I think Nate had it down just right for the fun part. If you
want to sneak in some educational advancement, try to entice them into
thinking it's their idea to learn some new technique. Sound excited,
it usually works.