What material(s), beyond human skin, can be used to interact with a
synaptics touchpad and specifically the iPod scroll wheel and buttons?
I have tried various types of materials and so far nothing beyond
human skin works.  A material like foil will work if you are holding
it with your bare hands because of the conductivity, but I am looking
for a [preferably] everyday or relatively easily obtainable material
that is capable of controlling a laptop touchpad/iPod scroll wheel
completely independent of human contact.

I know that these touchpads work of the principle of coupling
capacitance (see http://www.ipodlounge.com/faqs_more.php?id=1410_0_10_0_C).
 I have found the human skin has a dielectric constant of 33-44 as is
stated in the table at the bottom of this page: 
http://hypertextbook.com/physics/electricity/dielectrics/.  I believe
that a material with a similar dielectric constant to human skin
should work.

Very thick jello might do it ... It has been known to fool biometric
authentication devices on occasion...
http://www.puttyworld.com/thinputdeffi.html

Your results may vary. This is a free comment.

I have found that water works (as the body is made of mostly water),
but I am looking for a solid.

Ballistics gel? (A la Mythbusters)

It may not be the human skin, it may be the size of the human body,
try it again with the foil but this time connect something large to
the foil like a 15 foot wire, make sure the wire is spread out, if the
wire is crumpled up it will behave like a small object, please let me
know how it works out.

I have found the negative side of a AA battery(Energizer) will make a
touchpad move, not sure if this is the only type of battery or even
the same brand.