If you understand French, you might be interested in attending a theater play or a one man show. Two places in Paris offer half price tickets for the evening shows.
One is located on Montparnasse square and the other place de la Madeleine, close to the church, opposite to Hédiard.
To get an idea about running plays and shows, you can look at l’Officiel des Spectacles (0,35 euros) or Pariscope. You’ll also find, in both papers, films, concerts and exhibitions programs. (I prefer l’Officiel ). They’re published on Wednesday, because new films are released on Wednesday, but you can find either one throughout the week. It can help you make a choice between the night available shows.
Several websites also offer to book theater places at bargain prices, but not for the day. For example, http://www.theatreonline.com/guide/home_guide.asp?gclid=CILztZK8348CFQbclAodJDDF1g