I visited Myvatn for 1 day & 1 night, I wish I could stay half a week to see all its marvels!
This is a truly unique place, and although you can see so many things in less than 1h drive, my favourite place is the lake. Incredible landscape, perfect to wander in the nature.
My advice: if the weather is favourable (i.e. no cloud + sun activity), go on top of a hill next to the lake and watch the Aurora borealis - no photoshop needed :)