Focusing on the tone or intention:
* Entertaining
* Amusing
* Engaging
* Informal
* Lighthearted
* Informal
* Non-instructive
* Unpedantic
* Spontaneous
Focusing on the teaching style:
* Impromptu
* Intuitive
* Experiential
* Collaborative
* Informal
* Unscripted
Focusing on the content:
* Subjective
* Personal
* Creative
* Imaginative
* Non-prescriptive
* Open-ended
The best antonym will depend on the specific situation and the intended meaning.