Before you begin, you probably want to make it clear that this is intended to be fun: you don't want anyone to be offended. It's also an activity that probably works best in a class in which people already know each other to some extent, and get on well.
You could always steal your Mr Men from Google Images, but don't do that: instead, get your learners to draw them, by following these simple steps…
ONE Draw a circle, a square and an oval:
TWO Redraw them, giving them a "leg":
Believe me, it's easier to do ONE and then move on to TWO: experience with this in classrooms suggest many people struggle if you start with TWO (?!).
THREE Add features to your redrawn figure — noses, eyes, beards, eyebrows, hands, a second leg, props… whatever your imagination suggests, like these:
FOUR Decide who you've drawn, which must be someone you have some sort of relationship with (e,g. your mother-in-law, your husband, your ex, a self-portrait… but see Footnotes, below) and give him/her an appropriate "Mr Men" name — such as Mr [Silly] / Little Miss [Bossy].
Left to right, in my example above, you have my Dad; (the original Mr Grumpy); my sister (Little Miss Piggy — cruel, I know!); and myself (with toothache).
FIVE Show it to the psychoanalyst (aka your partner) who is sitting next to you.
SIX Have him/her "analyze" it and give a "professional" opinion.
SEVEN Discuss the opinion with your psychoanalyst.
EIGHT (optional) Class discussion of whether we can really draw any conclusions from such things.
With younger learners, you probably want to specify that they cannot draw anyone else in the class; or another teacher in your school, otherwise it can get cruel; with my own learners, I think I'd avoid mentioning Little Miss Piggy.
It's simple; it's fun; it's creative; it doesn't require Google Images (or much other preparation time); it doesn't require lots of talent (anyone can do it!); and — above all — it generates a lot of language.
Thanks @ Rachel B. for the suggestion that your learners can run their Mr Men characters into other activities, in order to illustrate other activities.