From Aikido Journal:

The founder of aikido himself obviously had a sense of humor, judging from the old movies where you see him playing a lighthearted "cat-and-mouse" game with his ukes.

A judo friend of mine experienced this at first hand in the early 60s when he was invited by O-Sensei to attack him "any way he liked".

Realizing he would probably be recognized as a judoka (the cauliflower ears were a giveaway) he decided to use the element of surprise, and to attack with a karate kick – but he never saw or felt what happened! He just flew through the air and landed with a thud. When he got up, slightly dazed, O-Sensei had apparently vanished! He turned this way and that, but could not see him – for the simple reason that the old man had sneaked in behind him and was turning around with him, in perfect sync, much to the amusement of the onlookers. It was only when my friend received a tap on the shoulder that he realized the joke was on him.

If that’s isn't a sense of humor, at the highest level of aikido, I’d like to know what is.