Improving at Excel without a proper reason to use it.

Hello everyone,

I'm taking a Spreadsheet Applications class for College Credit Plus because I like saving money, but I've got a couple problems. For one, there isn't enough classwork to make the lessons really stick. Second, I don't begin "actual" college until September (I'm going for Accounting), so I need a way to keep my knowledge fresh over the summer. Are there any personal projects I could start, or an online resource?

Any help is appreciated!