As you said "beautiful" and mentioned "calm" I'll keep my suggestions to the calmer sections. If you want uplifting and thrilling I can do more!
1st Symphony is lovely, the 2nd movement is breath taking.
I'm also a fan of by Kodály.
by Sibelius.
Also .
For shorter pieces, Passacaglia - Death of Falstaff from Henry V is just achingly beautiful.
from Falla's 7 Popular Spanish Songs.
Or his a Debussy (I prefer the original orchestral version, but there's a guitar version too).
2 Elegaic Melodies
And I particularly like the of Scriabin's 1st Symphony.
Arvo Pärt has already been mentioned, but I would recommend his
The first movement of his may have a less relaxing feel, but the second movement is tres relaxing.
For choral stuff there's loads of gorgeous music from the Renaissance;
For example Taverner's or Victoria's
Somewhat later, Bruckner's is stunning, particularly the opening Kyrie. I also love his (Pange Lingua is a particular favourite of mine).
And just as an extra, from Peri's opera Euridice.