My favourite calculator trick as a kid in the 80's(other than spelling
5318008 
) was to flip the polarisers so it reversed the display.... black background with silver digits.
The other kids always asked how I had a reverse screen but I kept it a secret
Made it a little harder to read in some light but it was well worth it for something different.
Can't do it these days as the polarisers are adhered to the glass.