Well, I have recently got ahold of nightwish's mini-album "Sleeping Sun (4 Ballads of the Eclipse)", and as relaxing it is to listen to those 4 songs it gets tiresome after the 2nd or third loop, I tried finding more songs similar to it and all I could find was their "Kuolema Tekee Taiteilijan".
So right now, my relaxing playlist consists of:
Nightwish - Sleeping Sun
Nightwish - Walking in the Air
Nightwish - Swanheart
Nightwish - Angels Fall First
Nightwish - Kuolema Tekee Taiteilijan
That's just to give you a reference of what I'm looking for, please post the names of any other similar songs you know, thanks :).