Is this supposed to mean that "I am coming back tomorrow" or that "I will return something tomorrow"?
There is no object, therefore it means 'I am coming back tomorrow'. Otherwise it would be something like 'I will return the book tomorrow' in which case you would probably use the future tense: Θα επιστρέψω το βιβλίο αύριο. The present tense is also used in everyday speech though. But you need a verb object.