Well... I think GoP would be a great idea, and so would be Bleak, but Atonement-Reverie/Harlequin Forest is not a good pick 'cause I think most of Opeth fans' population don't really like it very much and it is a hard song to connect to IMO. Though I personally do like it...
Umm... I also thought about taking The Drapery Falls, because it's a really awesome song and it has everything Opeth has to offer.
You can also use a cool instrumental maybe, which I can't help you pick though.
I think a clean-only track is necessary and I suggest something like Isolation Years or A Fair Judgement, but choose whatever you find likely to put there, this is just my opinion and probably my two fave clean Opeth tracks.
Blackwater Park might be a nice track to put there, 'cause you might also wanna show the growling part of Opeth too and, you gotta admit, the song is fucking great
When's a nice idea because it is also an Opeth great song and it shows really what Opeth is about, if you think about it.
Well, I guess you can just fill the others with your personal favorite songs, I'm pretty much out of ideas for other available tracks to put there.
Oh, and for the order you put them in, I think you might wanna start with a clean-only track and the rest is not very important - why? Intuition (which means I don't have a clue).
All said here is my personal opinion and you might disagree with it, I'd be happy to discuss my opinions here.