The short answer is, Everything!
A free trial account has exactly the same features as a paid account. ALL features are available and there is no limit to the number of roadmaps you can create. Also, you can add as many users to your account as you want, including Account Admins, Collaborators, and Reviewers.
Once you're nearing the end of you trial period (or whenever you're ready!), click UPGRADE NOW on the Account menu. To open the menu, click the Account icon on the far right of the Roadmap navbar.