The question seems like it may be opinion based, and it's hard to judge whether an answer is correct or incorrect, and it's more about language learning in general than specifically about German. But it seems like an interesting question regardless.
I've always been a fan of the way DW started it's radio/podcast German courses Deutsch – warum nicht? and Radio D. The focus of the first episode of Deutsch – warum nicht?, "This is a song", is simply to showcase the sound of the German language. They include samples from a radio call-in show, a nursery rhyme, a news broadcast, a song by Klaus Hoffmann, and an excerpt from "Faust" by Goethe. The focus of the first episode of Radio D is more about using non-verbal clues to figure out what's happening in a scene even if you don't understand everything that's being said. To me, one of the most important, and undervalued, skills to learn when you start to learn a new language is be able to "get the gist" of what's being said even if you don't understand every word.
I think something else to consider for a first class is how to get people motivated. I know many people take a language course for a job or to get a certification, but learning a language is a lot of work and I don't think external motivation alone will get you there all the time. So one question you might try to get your students to think about is what is unique about the German language or the culture surrounding it that will make it a language worth committing to for several years. For me a big factor is that it's a challenge, but not an insanely difficult challenge like Chinese or Japanese. German is or was the language of many great artists, composers, poets, writers, philosophers, filmmakers, scientists and mathematicians. On a less serious note, it's also the language of Grimm's fairy tales, Heidi, Bambi, Nena, Alle Kinder Witze, Ladykracher, Loriot, and whatever the hell this is.
Anyway, doing greetings and introducing yourself is certainly the traditional way to start a language course, and you won't be criticized for going with that. But I think there should be room for other, more creative directions.
On a final note, I have over the years made several attempts at learning German, not to mention a few other languages. What has made a huge difference toward whatever progress I've made this time is the tremendous resources available now through the internet. Need to look up a word? There are on-line dictionaries like Wiktionary and DWDS. Want to hear German spoken by native speakers? There's YouTube. Confused about grammar? There are many on-line Grammar references in both English and German. Stumped by some issue that doesn't seem to be covered anywhere? There's, well, this site. So I guess another thing to emphasize is that there are all these additional resources out there, and learners can make their journey much easier by utilizing them. You might refer to our FAQ for suggestions.