Many moons ago when I was teaching pre-school and pre-K I had a reward system which worked great for me and the kids loved it. Teachers in church run schools don't make a great deal of money so I came up with a couple of ideas to get great behavior and encourage good thinking. I had a "Light Bulb" award. It was nothing more than a light bulb, colored yellow, on 8 1/2 X 11 inch paper which said "I was very bright today". This was for good questions, solving problems, etc.
Another was a 4 scoop ice cream cone which said "I was very sweet today.", given for really good behavior and using good manners.
I also copied a page from a coloring book with a beaver. This one said "I worked very hard today". This was for helping to pick up, etc. These were put in the back packs on a daily basis for those who earned them. Sometimes I had several to give out and sometimes not so many, but they all understood that they had to earn them.