Our club operates primarily during the school year, as we are a school based club. We are blessed to receive donations from time to time and rather than distribute them willy nilly without regard to the students' interests, we hold a big student members only auction at the end of the year. Members earn auction money through participation all year long: meeting attendance, completing a project, giving a presentation, etc., all can earn them auction money. This encourages participation, makes it fun, and provides a good way to make sure the donated objects go to interested students... if they like it, they bid; if it's not their thing, they pass.
We don't use US legal tender for this so we create our own currency. This year we learned about military currency from Fred Schwan who visited us in October. Knights are military; so we borrowed the idea (and this year borrowed a British military currency design) to make our own "Knights Payment Certificates", or KPC. Each year these notes are devalued and replaced for next year's auction.
For an added twist, because we are Knights, and because the sun once never set on the British Empire or its currency, we denominate it in predecimal British pounds, shillings, and pence. This adds educational value as they will invariably see these denominations if they collect any world currency. It's tricky to bid with at first, but they catch on pretty quickly and are bidding like mad. (It helps if you feed them cookies during the event... it keeps the bidding energetic!)