Get the Metro to Mabillon or St Sulpice. Walk around to rue des Cannettes. Around here, there's 3-4 streets packed full of friendly, informal small restaurants and brasseries (with very good prices, too - you're looking at set menus around €15). Almost all are excellent. Particularly recommended is the Séraphin, at 5 rue Mabillon (www.seraphin.fr). Book ahead.