A mobile/web app maybe meteor stack w/ cordova if you wanted to go cross-platform. I'd build a calculator to end all calculators -- normal basic calculator, but

A mobile/web app maybe meteor stack w/ cordova if you wanted to go cross-platform. I'd build a calculator to end all calculators -- normal basic calculator, but have tabs for other types of calculators like Mortgage, Retirement Savings, etc... Basically take every type of financial calculator and bottle it up into a single app.