This pen looks fun: the Turtle Work and Turtle Play. The giant pens feature their patent pending SNAPback™ mechanism, which you'll have to see to understand. Both pens run for about $50 ...

Turtle’s decision to call the extended nib a “turtle head” is highly questionable, but hey—it looks pretty neat, though we can’t make any claims as to how well the thing writes.

Eventually, you'll be able to hold 40 Turtles in a single Turtle Pen. All told, this is an incredibly complex feature in writing, but looks to be fairly straight-forward in practice.