What should be put in the final slide when preparing teaching presentations?
Take a cue from TV. Pretty much every serial show I watch ends with "next time on..." It is simple, not prone to misunderstandings like trying to be funny, and actually serves a purpose of indicating what the students might look at before they come in for the next lecture. This is how I wrap up my lectures even though I do not use slides in the classroom. (I like to juggle multiple columns of coexisting text on a blackboard instead.)
You could consider making your last slide a reminder of what are the expected learning outcomes of the lecture, possibly with a link to other past, or future learning outcomes.
e.g.
In this seminar we looked at:
- Differentiation, from first principles,
- A graphical representation of differentiation, and
- The general formula for differentiating a function f(x).
Next time:
- Differentiation of trigonometric functions.
Usually, a 'this is what you should have just learned' bullet point slide not only sums up what your audience has learned, but also reminds them about topics/questions that have come up during your presentation. This is a great way for them to remember and for you to guide questions/following Q&A session along the talk.
Another approach that I personally like is a collection of further resources (or sources) that the audience can look up after your talk if they are interested into more details (or simply didn't have the courage to ask questions).