What form can informal reward and recognition take and when it might be applied?

Peer driven rewards are best.  You may set up programs where teammates and co-workers recognize others.  This recognition can be accompanied with a certificate of recognition and modest gifts -- such free meals, gift cards, etc.

This is one case where the thought is a larger gesture than the monetary reward.

Informal rewards are a very important appreciation geister to be awarded to uniqe or outstanding employees, this might take place for employees whom show a high level of punctuality and/or people that come up with new creative ideas that enhance the work environment.

Such appreciation might be through doing a gratitude gathering to deliver it or even through holding an awareness session showing this acheivement and might be picked as the star of the company. 

sometimes a 2 persons travel tickets might be gifted for the acheivers as an appreciation token from the organization.

Thanks.