The unsuspecting kid starts to reach into the bowl and a
disembodies hand pokes out from the bowl and hands them
a candy bar. Screams, mirth and merriment ensues.
Fake hand holding one side of the bowl, real hand on the
other, the other real hand in a hole at the bottom hiding
out the candy.
For grownup house guests that are friends with a good
sense of humor, you could also grab their hand as they
reached into the bowl.
A non-Halloween variation of this could be a party trick
where you play a game of "What's inside the box". Instead
of a bowl of candy you have a box with a curtain on it. The
person reaches in to see what was put in using only touch
and the perpetrator grabs their hand.