You can do some or all of the following:
1. Contact Manager
- To manage your friends, family, and business associates contact information
2. Guestbook
- These were popular once, but I don't see them as often anymore (thankfully), but it is still a pretty easy app to start with.
3. User Login
- Most websites these days have some form of user login.
4. Contact Form
- For customers to fill out. Either store the data in your database a retrieve it, send it via email using PHP, or do both!
5. Simple Chat
- Putting your User login script together with this to create a login area and then a chat area storing chat information in the database and then displaying it either in an iframe with a page refresh on it or by making AJAX calls.