How would I write a brief appreciation of Sophocles' Oedipus at Colonus?
This is a good question, because the genre of "appreciation" is one that was popular some thirty or forty years ago and is rarely assigned on the university level now, and thus not familiar to many students.
An appreciation is an essay which starts from the presumption that the work under consideration has some unique value to the reader and attempts to draw out that value. It often includes elements of reader response and is more personal and impressionistic than a critical essay. Generically, it falls under the category of "belle lettres" rather than scholarship, and should be written in a graceful style, using some figures of speech or other literary devices.
For Sophocles' Oedipus at Colonus, you might look at the redemptive nature of the work in contrast to the pessimism of the other Theban plays.