On my radar, there's only two categories of lenses that can be considered fun to me - Fisheye and Tilt shift. All others are serious fun, and no way close to light fun.
Firstly, our Nikon D90 already has a fisheye retouch function built-in and can practically turn any normal image to a fisheyed image. So, that's already half the fun there. So, unless you find fun in exaggerated perspective, fisheye lens will be fun in her own ways.
Now, don't think for a moment I can afford the Nikkor PC tilt shift lenses! They practically can cost my right arm and left leg to bring home...ouch! I am talking about a cheaper alternative, something in the zone I can call fun...
