What an fabulous idea! It would be fab for topical keywords as well. I am adding this to my list of resources.
Throughout the week, students add new vocabulary to the tube (or box or wall) and on Friday it’s emptied and used for revision any way you like. E.g. use this word in a sentence, define the word etc.
You could use different coloured card for different parts of speech. Green for verbs, orange for adjectives, purple for idioms/phrases and so on.
What do you think?