Have you ever had a day when what you wanted most was to laugh? It sometimes happens that thinking through problems and solutions is overwhelming and grinding out work is exhausting and the only real solution is to laugh. This is hard for introverts, since we laugh more easily in the company of others (we all know that laugh tracks work because the quickest way to get someone to laugh is to laugh yourself).
Several times a week, I spend time with my laughter coach. She does not charge by the hour, but she does value obedience and full engagement. My coach is my 18 month old niece. She is a giggle expert, but she has also mastered the grin and the chuckle and the bemused half smile that indicates she can sometimes suffer fools with good humour.
Although spending time with C means lifting and moving and sitting on the floor, my back is strangely compliant while I am with her. Some of this is the value of love and some comes from the therapeutic touch of small arms giving a big hug. But I am convinced that much of it comes from nearly continuous smiling, giggling and laughing out loud.
If you don't have a laughter coach, perhaps you will have to settle for games that surprise you (I have also taken up racing video games) or good-natured comedy (somehow small-minded does not produce the right kind of laughter). Stories with reliably happy endings are also a good bet. You don't want to be engaged so much as you want to condition yourself that life holds the kinds of surprises that result in giggles.