The 'hop on, hop off' Batobus river service is far cheaper than the river boats that have guides on board them and it's a great way to get around many of the best sights:
http://www.batobus.com/english/
The Musée d'Orsay is well worth a visit:
http://www.musee-orsay.fr/en/home.html
The Latin Quarter is one of the best areas to eat cheaply. A really good 3-course lunch costs around €15 (but there are even cheaper options available too).
I enjoyed the Le Musée des Égouts de Paris, which is a self-guided tour of the sewers!
http://www.paris.fr/english/heritage-and-sights/main-sights/visit-the-paris-sewer-system-duplique/rub_8275_stand_132295_port_19140