The concept is a quiet evening in the Fire Nation with Zuko playing the liquin/chinese lute that belonged to Uncle Iroh while Toph fishes. Its an opportunity for them to unwind and relax, loosely reminiscent of the campfire nights of old during the series (which at this point would be about a decade ago).
Something to note is that since Toph refuses to spend any length of time in a boat, she ended up earth bending a path to a good fishing spot.
Also, Zuko has a few sets of clothing for going incognito - including an Earth Kingdom outfit so he can sneak out amidst dignitaries/diplomats when leaving palace grounds.
click for full view
Close-up details: Zuko | Toph | Feet | Instrument | Engraved gauntlets | Background
Progress shots can be seen here!
The post on DA is over here :D