There is a plugin for Rhythmbox for browsing directories
One annoying thing with the otherwise capable Rhythmbox for Gnome, is that your music library becomes one big soup of songs. However there is a plugin for Rhythmbox that allows you to browse your directories. It is called folderview and allows you subselect from your music collection based on path, which means that you can browse in a tree to a directory within your music repository, and folderview will display and play all songs in that directory and its subdirectories.
When I say "will play" that is exactly what it does, it will immediately switch to and start playing the first song it finds, if it detects that Rhythmbox is already playing something. Besides this minor quirk it is a very useful plugin: Kudos to the author and to the architects of the Rhythmbox plugin system!
If you want completely separate entities under Rhythmbox, with separate playlists, take look at the mt-daapd (Firefly) DAAP server in conjunction with the Rhythmbox DAAP plugin.
Rhythmbox folder view plugin