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Way back when I had a guild on an old account. I did a contest that was great. It was a monthly drawing. The prizes I offered were monthly letters but you can offer any thing really. It took 2 threads, 1 was the sign up and number list, were players that wanted to be in the drawing would post ONE time, then a mod would go through and edit each post giving each person a number. Because the expiation took up the first post then each post after that was players and there number 1 post behind, IE, post 2 was number 1, post 3 number 2, see. Easy to keep track make sure no one changed there number. Then the second thread has the over all explanation, and the drawings. The mod each month looked at how many people were signed up, say 25 were, they set a post to pick a random number between 1-25 and the winner is the person with that number. I did not charge for entry but I required people to post say 20 times in the thread that were not bumps before they could enter. That way they were at least a little active.

A second game I did, also offering letters but again any prize will work, Lucky Number, pretty much the mod made a list of prizes.

prize 1-#50
prize 2-#66 ect...
then each day each guild member could role a random number from 1-100 and try to win. First person to roll the number matching a prize gets it. One entry per person per day.

Another game I did, and it was my favorite was Find the Mimzy. First I edited the image for the mimzy making like 10 different background colors. Each color was worth a prize. Then I went through the guild and edited posts placing the image of the mimzy into a post. People looked for them. Once they found one they got the prize. I was careful to ether put them on page 1-3 or one of the last pages in a thread and in no threads that would be very very active. The harder the mimzy was to find the better the prize was.