I love everything Beethoven. Just when I thought I've listened through the symphonies and sonatas, a friend recommended Beethoven's quartets to me. WOW! I daresay that the quartets are Beethoven's most personal works. Anyway, the quartets are relatively relaxing; however, some movements can be extremely mesmerizing (particularly the slow movements in the Razumovsky Quartets and the Late Quartets).
If you have time, check out all of Beethoven's Quartets.
Start with these:
String Quartet No. 7, 8, 9 (Razumovsky)
String Quartet No. 10 ("Harp" Op. 74)
String Quartet No. 13 (Op. 130)
String Quartet No. 14 (Op. 131)
String Quartet No. 16 (Op. 135)