October 23: MercPerks coupon: 50% off $20 or $40 at Next Adventure
 

This is similar to Groupon, but they're typically available to purchase for a week, not just one day:

http://post.portlandmercury.com/portland/Shop?oid=1980902

You can buy a $20 voucher for $10, or a $40 voucher for $20.