You need to get the number playlist elements and then query for each one to know its object_id:

{
	size = mafw_playlist_get_size (playlist, &error);
	if (error != NULL)
	{
          /* Error management */
	}

        for (i = 0; i < size; i++)
        {
          gchar *oid = mafw_playlist_get_item (playlist, i, &error);

          if (error != NULL)
	  {
            /* Error management */
	  }

          /* Information treatment */
        }
}

If you want more information, like metadata associated to a particular object_id you have to use the asynchronous API.

As in the last example, first you need to know the playlist size and then query metadata for a range within the playlist limits. Usual use case could be from the beginning to the end of the playlist:

{
	size = mafw_playlist_get_size (playlist, &error);
	if (error != NULL)
	{
          /* Error management */
	}

	mafw_playlist_get_items_md (playlist, 0, size - 1,
                                    MAFW_SOURCE_LIST (MAFW_METADATA_KEY_TITLE),
	                            playlist_get_items_md_cb, NULL, NULL);
}

static void
playlist_get_items_md_cb(MafwPlaylist *playlist, guint index,
			 const gchar *object_id, GHashTable *metadata,
			 gpointer userdata)
{
	GError* error = NULL;
	GValue* value;
	const gchar* title;

	value = mafw_metadata_first (metadata, MAFW_METADATA_KEY_TITLE);
	title = g_value_get_string (value);

        /* Probably show this title string in the UI */
}

You can see the metadata is returned in a hash table that can contain metadata with multiple values and this is why MAFW provides mafw_metadata_first to access the first (or only) element in the hash table for a particular metadata key. If you want to get more values you could access directly the hash table. If there is only one value, the hash table will return a GValue, but if it is multi-valued, it will return a GValueArray. Take a look at the Mafw API reference for more info.