There isn't as much use for old-school four-function calculators anymore since we all have them on our cell phones now. If you've still got one lying around, you can hack it to play recorded sounds, use it to make a metal detector, or turn it into a custom name plate that says anything you want...
Tom over on Magic Smoke inserted a 50-pin 6 digit LCD display into a cheap calculator and hacked it to flash his name by messing with the pins. By turning different segments on and off, you can make it display any message you want.
It requires a lot of experimentation to learn which segments need to be on and off, but it's more time consuming than difficult. You can find a circuit diagram along with more details over on Tom's blog.
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