So long and thanks for all the fish.becomes
Σο λονγ ανδ θανκσ φορ αλλ θε φισh.which would be quite confusing to a Greek speaker but perfectly natural to an English speaker who knows the Greek alphabet.
Note that some English characters do not have direct equivalents in Greek. A c would be a κ, a σ, or a χ for ch. The h is one of the more interesting stories. For a leading h before a vowel Greek uses a breathing mark. When combined with a consonant, special characters are used, like θ for th, χ for ch, and φ for ph. I chose to use φ only for the English f, not ph, since ph does not make the f sound in some English contexts. The word uphill is one example. Also, I didn't bother to handle the special case of s at the end of word, for which ς is used instead of σ.
Now that you know more than you ever wanted to about the Greek alphabet, what is a simple project that you can tackle in an hour? Don't just think of one, go do it!