Given a small bag containing a marble, a feather, a rubberband, and a
stick of peppermint gum, what product could you quickly create? How
would it be positioned?

Choking Hazard Activity Set (TM)
Market it for kids.

MacGyver Nuclear Reactor Repair Kit.

Id make a slingshot of it

Fast food "Happy Meal" for goats?

Break the rubberband. Tie one end to the feather, marble and gum. 
Position it for sale outside Roman vomitariums.

A breath refreshing solution for male pattern baldness for geese?

But seriously folks . . .

I think the assemblage as you described it (a bag containing all those
items) could be instantly marketed as a kid's "Guessing Bag" or a
party favor "Guessing Game."  Place ten of the bags, with the enclosed
items, into a larger package, slap on a colorful label that reads,
"Guessing Game Party Favors!  Give each guest a bag and the one who
correctly guesses the contents by feeling through the bag alone wins a
prize!"  Position them in stores that sell kids' birthday party
supplies.  All the kids get to keep the bag and contents and can
re-use it (assuming the gum remains unmasticated) to tantalize and
torment siblings at home.  Each item is tactilely distinct, and you
inherently have varying degrees of difficult, from the marble (easy)
to the feather (difficult).

(I think I have talked myself into doing this as a little start-up business . . .)

For a Bet ... 

That posing this Question on GA for a mere $2 would tickle the fancy
of some of the most respected Researchers in the GA-laxy (especially
Pinkfreud and the somewhat fishy Denco) and excite at least 8
Comments.

Mohican Super Laxative remedy (TM)

Market it to Mohicans.

Easy!  You can use it to market the EAGLES.  Put team player's faces
inside the marble.  Put interesting trivia about the EAGLES on the gum
wrapper.  Use an eagle feather to stand for the EAGLES.  Make that
rubberband a hair-tie for girls to thinks the EAGLES are hot.

Pretty easy answer for ya!