Now, creating a prompt sounds way easier than it is. While I had some fun ideas I had a hard time judging if other people would also think them fun. There was also the issue to solve of wether to just give one prompt for everyone, let people choose, maybe a vote by hands? In the end I decided to ask some people that would not be participating in the event because the had plans that day about their opinion on the development of the prompt, since I did not want the participants to know before the event. In these conversations, it became obvious that people did want to have a little bit of a choice about what to do, especially since the prompts were not only different in content and “vibe”, but also discipline. So giving a poster design prompt might annoy people, even if the topic of the poster would be interesting.
So in the end, I created a google form as a kind of sign up which helped me in two ways: I was able to gauge about how many people were interested in coming, which was important for the setup, and I could ask people their opinions on what they were interested in doing, discipline wise. These were the results:

So most people wanted to do a random graphic asset, also guessing that they wanted to be surprised by something that was further from typical classwork, and second in line was logo/branding.
So the final two prompts given at the event were:
1. Create a branding/logo for your favourite conspiracy theory
2. Design a birthday card for david (a friend of mine, its his birthday today)
Out of the 12 people that came, this was the ratio:
branding of conspiracy theory: 6
birthday card (so, random graphic asset): 3
The remaining 3 people did not participate in the prompts, and used the space and time to work on different projects. So this was also a clear learning for me, that there maybe need to be more rules than I had, to make sure people participate, while not feeling pressured.
Some of the discarded prompt ideas (in case anyone is interested) were:
wedding invitation for 2 clouds getting married, redesign the periodic table, business card for a ghost who freelances, type poster with a font you hate, “knickbild” plakat (folding paper, handing it around, unfolding at the end and using the result as a base for a design), redesign currency.