Once the game is over, everyone gets to eat the contents. You could also use pennies, dried beans, screws, soft drink tabs, paper clips, even confetti (really tough). Try to think of something that would be the most intriguing to your probable participants. However, unless the items are donated or loaned, keep them very cheap for obvious reasons. 

Now that your jar is filled, you need to place it in a conspicuous place such as a display case at a school or break area at a business so everyone can see it. Place a sign next to the jar (and in all other areas you can think of) challenging participants to guess the number of "whatevers" in the jar. Your specific organization and target participants will dictate how much to charge per guess and how to collect for the guesses. Make sure your signage explains what the proceeds will be spent for.

There are many fundraising variations that can be considered. Have a guessing game jar at each of your monthly meetings with the proceeds going to a specific program or activity.  Eliminate the jar contents and have the guess relate to an event - the closest time to a member's baby being born, the first snow fall of the year, the first day the Dow will close at exactly xxx, etc. Let your imagination run wild!