iTunes syncs each device individually, and allows you to have different settings for each device. If it were me, I would create a “Relaxation music" playlist and set it to sync to your iPad, "Party Music" playlist on your iPod, and such. Then plug your iOS device into iTunes (or use Wifi), and make sure “sync only checked songs and videos” and “Manually manage music and videos" are NOT checked under Summary > Options. Then select the Music tab, and check “Selected playlists, artists, albums and genres” deselect everything except the playlists/songs/albums you want on that iOS device.
One thing to note: you can create other playlists that divide "Relaxation music" more and then set those to sync to that device as well.
Repeat the above process for every device - setting only the desired playlist to sync to that device. In the future, you can easily change what you sync to each device by just changing the music in their perspective playlists.
