What are some sentences that only contain one word?

No such thing in the English Language. A sentence must contain a subject , verb and object. e.g. I saw the aeroplane.

'I' is the subject

'Saw' is the verb (past tense of 'to see'.

'The Aeroplane' is the object.

If the sentence does not contain these components then it is not a sentence.

NB Saying 'bye' or 'goodbye' on parting is NOT a sentence, but a simple statement.