I think this trick is easier than the easiest you have posted. =)
it's a stumper and a lot of fun with no prep.

The trick:

    A towel is rolled up like a rat-tail and placed on the bar in front of your victim (can also be done with a piece of string, however, in a bar, towels are generally more accessible).  

Said victim is told to tie the towel in a knot with the following stipulation; once the towel is picked up, he/she is not allowed to let go with either hand
until the task is complete.

The solution:

    the trick is to cross your arms before picking up the towel. See pictures. (Thanks for the pictures pdub)

hope you like it.

-pdub