Something that I used on my PGCE year that I had kind of forgotten about was plenary postcards. I have made a stack of these today but haven’t actually used them yet so I’m yet to see if they work well at the school I work at now, but the pupils at my placement school loved them!
An easy way to include a plenary into your lesson is to give one of these postcards to each pupil. They have to explain something from the lesson, maybe something in particular that you ask them to write about, or it can be just reporting something they found easy/difficult/interesting. There is also a space for an emoticon in the stamp space in the corner, which pupils particularly enjoy.
I have seen a little 50com post box in Hobbycraft that I think would be a really fun way to collect them in. I often used them as an alternative to an exit ticket, and collected them in at the door, but maybe having a little postbox or some way to even stick them on the wall would make them more fun?