It is a very bad idea to allow anyone to be an admin of a forum, even if it’s just for one day. Admins have access to everything:
- Any personal information of users, like email addresses.
- Any site setting, so they could just disable sign up for everyone else.
- The whole theming interface, so they could enter malicious javascript code.
- …
I wouldn’t want to visit a page where this is possible for everyone!
If you want to test all options of Discourse, you need your own site (dev install, self hosted, free tier of Discourse Hosting, free trial of a higher tier, …)
Yes, there is some default content to interact with. That is more helpful than a blank forum.