Add a downvote button that does the opposite of what the upvote button does
Sort topics by their number of votes
Add an 'about' page, linked from the bottom of the Suggestotron topics list. Link back to the Topics list from the About page so users don't get stranded.
If your class has a lot of time left over:
Users should be allowed to vote only once: give votes a user_id and allow a user to have voted on each topic only once. But wait, what is a 'user' in our system? Get a volunteer to introduce everyone to Devise, a simple way to add authentication to a Rails application.
Users should be able to give a post a 'negative' vote instead of a positive one. How might you represent that in this system?
...and many, many others
Probably time for the closing presentation.
After that, start a project, tutorial, and come back again!