Brahms transcribed most of his orchestral and chamber music into four-hand piano arrangements, a form in which most people got to hear and play those music during Brahms' lifetime. It is very interesting, and often revealing, to hear his texturally dense orchestral works in the striped down piano version, The first piano was actually planned as a sonata for pianos. No.15 of the Op. 39 Waltzes for piano four-hand is one of the most lovely tunes that Brahms, or any composer ever written. There are many gems in the WoOs and transcriptions in other forms too. Check out the previous playlists of Brahms' works with opus numbers if you missed them.
Here's the Spotify playlist:
Brahms: WoOs and Transcriptions (228 tracks, 12 hours)
Brahms: Four-Hand Piano Music (167 tracks, 13 hours)