Didactic poetry can be humorous (see again, those Sesame Street songs, which are brilliant!) or somber.
Didactic poetry can also cross a(n unseen) line and become Ars Poetica (a poem about poetry/ poetry creating). And in point of fact, that has a long historic tradition as well, starting with Horace's Ars Poetica (English translation here). Most of my didactic poems are also Ars Poetica, because I tend to think meta-poetically. (Remember my poem from last week after I cut my hair?)
But the only real necessity for a didactic poem is to write out instructions. First this step than that (or more subtly, a series of instructions that flow into each other).
So you see, you can pick any topic to instruct us on in your poem today. Gardening, housekeeping, funeral making, cutting hair. Or you could sweep into the political or philosophical realms. Subject matter is up to you.