How do I mark a card as done in a retrospective?

If a retrospective card represents a to-do item, it can be useful to mark it as done when it has been completed. There are several ways you can do this.

One way is to add a tag on the card, for example a green "Done" tag like this:

Another way is to just change the color of the card, for example to green:

Finally, you can add a checklist in the card. The number of checked items will then be shown on the card, like this: