Students absolutely love QR codes or maybe they like them. It love getting to use their own device. Either way, they get moving and stay engaged for an entire class period and that's a WIN!
I created 10 problems (with 10 correct answers), 20 incorrect answers (I used the same digits just moved the decimal points around). Then I used a QR code app to create the codes for my problems. Here are some pictures from the app.
(Above) I typed all of this in the app and a code was created. When it was scanned this was visible to my students. They had a recording sheet (I'll attach later) they wrote the problem, worked it, and then looked around the room for a poster with their answer. When it was found they had to scan the code and get an oops or correct message.