Engage in play

Join a Laughter Club.
If laughing every now and then is not on your immediate list, you should definitely find and join a laughter club. A lot of these events are also organised online nowadays.

Visit places that are high on humour or joyfulness.
Schools, playgrounds, and parks have children - seeing them engaged in their mindless innocent activities might just make you laugh heartily. Let your adult immune system be attacked by the playfulness these places have to offer.