If you want to make it interseting and involve as many people as possible.
The contest should be 7 days long and anyone who involves themselves in a certain pre determined event gets 1 card for the day. On Monday you can make a list of things to do in order to qualify for a daily card. If a member misses a day or 2 they can still have a chance to win a prize. This eliminates the advantage some may have if they have the ability to collect more than 7 cards in a week.
Key to success is 7 card max.