What are some simple wrist/arm strengthening excercises I can do anywhere at anytime?

[Cute cat 25]

I have just begun a Hospitality course and this Friday I am doing my first shift at the training restaurant on campus. Whilst we were practicing carrying three plates my arms became so sore and the teacher suggested to the class that we do some strengthening exercises. Does any one have any simple exercises I can do, anywhere and at anytime that will strengthen my wrists/hands?
Also my wrists always click and when i squeeze a stress ball to try and strengthen them they still click is this bad?