Of course this all depends on the tourist's level of gourmandise and their pocketbook. If they are easy to please then try anyplace with French people in it -- usually away from the big boulevards.

An old, very touristy standby is:

CHARTIER -7, Rue du Faubourg Montmartre, 75009 Paris, France - 01 42 46 86 85 Very reasonable, there's nothing on the menu over €11 and entrees are as low as €2 or 3.

If they are more difficult to please, try:

COUDE A COUDE (elbow to elbow) 46 rue Saint Honore (Metro Les Halles, Louvre, Rivoli ) This is part of the 'slow food' movement, food from small producers, unusual plates and cheeses you don't see every day like Andouille without the offal taste; Good wines. Crowded so go early. Lunch formula is €12.50.

or for more nouvelle cuisine:
LE RESTAURANT at the corner of Rue Veron and Rue Audran, 18 Metro Abbesses or Blanche. An egalitarian restaurant packed every night where all entrees are €9 and all main course plates are €18. Rabbit with Juniper Berries. they have a €15 dejeuner formula and a €22 dinner formula. www.lerestaurant.fr