I decided to make these since I couldn't find anything for high school chemistry that related to the NGSS.
Directions: Print and cut out all 12 NGSS applicable standards for chemistry (physical science and engineering standards)
Once students have mastered the concept, give them the appropriate card. Make it more challenging by providing them with the list of standards (included) and at the end of every exam have them write which standard they think they've mastered. If they pass with a "C" or better, reward them with the NGSS card.
If they redeem all 12 or however many you're happy with, allow for them to redeem for extra credit or one replacement grade for an assignment (whatever reward you decide).