Any of these programs from the list would be good for a portfolio. You would have to make them full featured professional looking applications and extend them beyond what is given.
- Movie Store
- Airline / Hotel Reservation System
- Bank Account Manager
- Patient / Doctor Scheduler with multiple doctors
- WYSIWG (What you see is what you get) Editor for web pages
- Web Browser with Tabs
I didn't think those projects would be unique about. I guess I should stop assuming