We use PHPBB extension
Show First Post Only To Guest for that and more. Basically for guests, they see the topic and up to a number of characters of the first post of that topic, but no posts after the first. They must login as a registered user to see what everyone in that group sees. In this way, it urges guests to create an account, in the hopes that since they're already logged in, they'll ask or post feedback, which only registered users would typically have the privileges to do.
Number of characters to display:
Enter the number of characters to display for the first topic (default is 150). Setting the value to 0 disables this feature.
Each forum has it's own settings that can be adjusted to suit. PHPBB forum permissions override SFPOTG, so only the forums you already show to guests would be affected, i.e. adjusting SFPOTG settings for permissions restricted forums is wasted effort.
If you want to have some unrestricted days or weekends, you can disable and enable the extension. It's quite flexible that way.