I like the the sauna tube idea the best(cheap and fast). Have them (6) come up 6" above grade and anchor your 6x6 p/t sill plate to the top. Also you will need a mid-height girt for a nailer at least. A 6x6 frame with 6x8 ties with though(or extended) tenons should hold things together just fine. Sounds like a great beginner project to learn from.
Thane

Life is short so put your heart into something that will last a long time.