For now, no one has asked me for anything. They have simply asked me. The thing is that I didn't know how to give a concrete answer, because I have never done this before (and they do get to do some strange things with networks, but not this).
So it's something that's more or less widespread, the thing with cafes with wifi, but it's something that I don't know how to solve.
So in principle, to keep it simple, it would be enough to have one normal wifi and another for guests.
Can you password protect the guest one? Can you make that password expire? Can you give the network a different name?
Because, to start, that would be a good approximation. What router do you use?
Then it would come to "if you spend €5 you have 50 minutes" (for example) or a download of X megabytes...