Here are some great resources I've found so far:
"Projects About Ancient Greece" by Marian Brodia
The kids need more than just stories and written work- they love hands-on projects! This author has a plethora of books with activities related to different time periods. These books are great because not only do they have really easy, understandable projects, but also provide a bit of background history for each activity. These activities are great jumping-off points for more in-depth study.
Today, for example, the kids recreated their own version of the Minoan Bull-Leaping Fresco found at Knossos Palace on the island of Crete. This project inspired us to look up videos of modern day bull leaping (wow!) and gain a deeper understanding of the inspiration for the fresco. (And how could we do all this without talking about the "fresco" art style?!) We also talked about why the background of the original fresco is blue. Perhaps because Knossos is on an island? (One darling added fish to her fresco-recreation- she said island people would probably have thought fish were important-love it!)
We tied in a myth by reading the story of "Theseus and the Minotaur," which takes place at Knossos Palace. The illustrations were so great- one page has a repeating boarder of the bull from the fresco!
After our story, we discussed whether or not Theseus was a hero. The consensus was yes, any guy who goes to fight a dangerous bull/man beast to save peoples' lives is definitely a hero.
When I asked the kids what they thought the truth was behind the minotaur, though, they were stumped. This question lead to a great discussion- how the myth displayed the importance of bulls and bull-leaping in the Minoan society, how stories get distorted as they are passed along orally, and what could have lead to the idea of a minotaur.
I hope this lesson has given you some ideas for your own adventures in Greek mythology! I'd love to hear about similar lessons, extensions, or resources you've used!