For today's project, let's make a clone of Twitter.
[+] Start off with user signups. When somebody signs up, send them a welcome email.
[+] Then, let users post tweets. If another user's handle is mentioned in the tweet, send that user a notification email.
[+] Now, let users follow each other.
Add pages showing who a user follows, and who follows them.
Notify somebody by email if when they are followed.
Let users upload photos in their tweets.
Add pagination, or the splitting up of a big list into several pages, to your index views, using Kaminari.
Create a preferences page where users can opt out of different notifications.