The Louvre is open late so that is my suggestion.
Here is a list of things open. Should start looking pretty familar by now ;P
*Cath�drale de Notre-Dame: Cathedral 8am-6:45pm. Towers and crypt 10am-5:15pm. Treasury 9:30-6pm. Museum closed.
*Arc de Triomphe: 10am-10:30pm
*Mus�e du Louvre: 9am-9:45pm today and Friday
*Mus�e d'Orsay: 9:30am-6pm
Tour Eiffel: 9:30am-11:45pm; stairs open only to 6pm.
Cimeti�re du P�re-Lachaise: Hours: Mon-Fri 8am-6pm; Sat 8:30am-6pm; Sun 9am-5:30pm
H�tel des Invalides/Napoleon's Tomb: Hours: 10am-5pm
*The Panth�on: 10am-6pm (last entrance 45 min before closing)
*Mus�e Picasso: Hours: 9:30am-5:30pm
*Mus�e Rodin: Hours: 9:30am-4:45pm
Maison de Victor Hugo : Hours: 10am-5:40pm
Mus�e du Vin (Wine): Hours: 10am-6pm
Mus�e de l'Erotisme : Hours: 10am-2am