Are there any CiviCRM loyalty/rewards extensions available?
This may not be exactly what you were after, but CiviCRM does have the ability to add "premiums" to contribution pages as a reward to donors. For example, "Donate $20 and get a coffee mug".

From the book:


  Premiums
  
  Configure premiums, such as T-shirts or subscriptions, that you want to offer on your contribution pages:
  
  1) Navigate to Administer > CiviContribute > Premiums (Thank-You Gifts). 
  
  2) You can edit an existing premium or click Add Premium to add a new one.
  
  3) Once you edit or add a premium, you can then enter additional information: Name, Description, SKU (an optional product code), Premium Image (an optional image of the item), Minimum Contribution Amount to receive the premium, Market Value of the premium, Actual Cost, and Options (e.g., colors and sizes). 
  
  4) If you're offering a subscription or service, you can also click on the Subscription or Service Settings and define additional information here, such as Period type (e.g., Fixed or Rolling), the Fixed Period Start Day, the Duration, and the Frequency.