The typical standbys:
Inflatable soccer ball, Frisbees, Transformer-style toys, small racing cars, a small kite.

Toys that might be suited for more introspective kids:
An origami kit (paper and instructions), a collection of legos, a paint/calligraphy kit, binoculars/small telescope, microscope/magnifying glass, Rubik's cubes, puzzles in general, model kits, sand-box style toys (stuff like legos where you can do/build a whole variety of stuff instead of being constrained to just doing one thing).

lol, I forgot about those.

Those metal balls and magnet connector things are awesome.
When I was little, I used to spend hours playing with just a handful of those.  I carried them around with me all the time and would play with them in the mall as my parents went shopping.

I think there's a similar toy floating around that's just comprised of magnetic ball bearings?  I thought I saw it on ThinkGeek once.

Magnets in general might also be cool.

Non-electronic toys you can find on ThinkGeek might also work -- they have a bunch of pretty neat thingies.