Personally, I think it should be music and not 'songs', words can have a lot of connotations for people and could defeat the purpose of your treatment, unfortunately.
I really like Ruth's suggestions, my daughter calls it 'plinky-plonky' music which is maybe not the best description of relaxation/therapeutic music:lol:
LOL at "plinky-plonky" music :green:
Talking of charity shops though, I managed to pick up a "Chilled Ibiza" double album, and a couple of Julee Cruise albums for only a few quid; OK the Julee Cruise albums were quite scratched, but "CD Paranoia" on my Linux box was able to read them OK thank goodness! The Julee Cruise albums were really mellow and chilled out too - am sure some of the tracks off them would be nice for treatments.
Other music that I like, that Sharon my therapist also likes for treatments (I introduced her to it), is by a group called Bliss,
Bliss, Blissful Music, Blissful Records - Lucinda Drayton, Andrew Blissett - some of their albums, e.g. "Through These Eyes" have quite a Celtic sound to them, "A Hundred Thousand Angels" has a mix of Celtic and Indian-sounding tracks, whereas "You" is a more up-beat album and has a more jazz-oriented sound.
Another artist worth a listen is Kitaro. He's a Japanese musician who writes predominantly instrumental music, using a mixture of synthesizers and acoustic instruments. Good albums to start with are probably "Thinking of You" and "The Light of the Spirit" (the latter is more varied in style, and has more powerful, exciting tracks as well as some nice mellow ones).
Other people you might like... Terry Oldfield (Mike Oldfield's brother) has an eclectic discography, including several releases on the "New World" label. A lot of his work is influenced by world music.
One of my absolute favourite albums has to be "In Unity" by Tim Wheater and David Lord though. It's a mixture of Tim's gorgeous flute playing and David's sumptuous synthesizer backing (lovely rich analogue-sounding strings etc). Not sure how you'd like it given your dislike of pan-pipes; it's a "proper" flute rather than pan-pipes though.
Then there are some albums by Vangelis that would work well for treatments too, e.g. "Oceanic", "Opera Sauvage" and parts of "Voices".
As you might have guessed, I'm a big fan of classical, New Age and electronica lol...