I have used various J. Greshem Machines "New Testament Greek For Beginners" and the workbook to go along with it. NOt easy without help (also koine) but very good.
Also I used "Athenaze: Book I" Very easy for attic but I didn't have an Answer key.
This Time I am Useing Pharr's "Homeric Greek: a book for beginners"
The first was the free download from Textkit
But I did go through the Textkit store and ordered the revised text by John Wright.
A very useable text. Easy to grasp concepts, and as far as a key: William Annis has done a real nice job and you can download it for free.
That's my two bits. Hope it helps.