for downtown music/theatre tickets, there is a booth called HotTix (like the one that sells discount tickets for Broadway shows in New York City). Here is their website in downtown Chicago: http://www.hottix.org/
you can also take your mom to the Blue Man Group. The above website also sells their tickets.
Or, a great play to see is Wicked. You can either buy tickets at the box office or you can try your luck at Borders (on the Weekends or at the box office during weekday)...they draw twenty tickets for each showing for $25/tickets. Here is informatin on that:
"That's because a ticket drawing is held two hours before each performance begins. Just show up at the theater box office, 24 Randolph St. (or the Borders bookstore at 150 N. State St. on weekends), and enter. If your name is pulled, you can buy up to two tickets in the front two rows for the next show. For more: www.wickedthemusical.com/chicago.
here is information on Drury Lane in downtown:
Similar drawings take place for "The 25th Annual Putnam County Spelling Bee," now showing at Drury Lane Theatre, 175 E. Chestnut St. For more: www.spellingbeethemusical.com/chicago.
Have fun.
L.