This would be really cool:
Give a $10-25 gift card to the person who spends the most amount of money on the shop each month. I know it won’t be me, but it will be cool.
Or you could just do Stick It Click It and win a $25 giftcard once a month.
I’m with owngry on this one.
I don’t mind entering a contest or two during the year…trust me on that. Adults like to be winners too, because they rarely get anything for free, and they usually buy things themselves. Those with kids, are busy buying for their kids, and hardly ever on themselves. It makes me feel good to give to grown ups too. Everyone likes to win something.
Now that I got that out of the way. 
Despite all that, I don’t know about making it a practice of giving the gift certificate to the biggest spender. I think it’s better to have a contest that everyone equally can have a chance to win, as long as they do what it takes to participate per the rules. No financial discrimination. 
Then, it’s more fair, not based on who has the most, spends the most, or who spends or has the least for that matter. It’s on more of a level playing field.
I love Click it Stick it because there are usually 2 chances to win every month too. Winner by votes, and a staff pick. Everyone rich or poor can snap a photo, so it’s on a level playing field. The contest is about the thought and effort in setting up a good shot using the sticker, so that seems pretty fair.
YoyoExpert takes very good care of the customers who spend the big bucks. 
They show their appreciation already.
Besides, TA would win every month.
I might be up there, but there are some more silent ones on here who have me beat by a mile.