Besides a portfolio site, which I think is a great place to hone your skills and get something of value from it afterwards, I would find something you're really passionate about. Even if it's been done countless times before, if you're passionate about what you're working on, it won't seem like "work" for you, and you'll enjoy it that much more. I've been involved in the bartering community for the past few years, and because of that, my latest and greatest project has been a customized community based barter and exchange web site, and it has been a blast to make!