Build a blog. Keep it simple:
Have a homepage with 10 most recent posts
- Post can have many comments
- Only users can comment
- Users must be logged in to comment
- Only you (an admin) can log in and make posts. Or anyone else with credential.
- Use SSL to encrypt your login.
Extra credit:
- Posts can have tags. Harder than you think.
- Comments can have comments. This is harder than you think.
Extra extra credit:
- Implement it once using an SQL database. SQLite/MySQL/Postgres will be fine.
- Implement it using a NoSQL database. MongoDB will probably be the simplest.