If you plan on doing several of the Falls attractions, I strongly recommend you consider buying the Passport to the Falls pass. On the American side, the Niagra Falls State Park Service's Passport to the Falls (tel. 716/278-1796), will gain admission to Maid of the Mist, Cave of the Winds, the trolley, the Aquarium, Discovery Center, etc along with tons of discounts. You will almost save 30% on all the above attractions when you buy the pass. The pass can be purchased at the Visitors center.
If you purchase the ticket on Saturday it is valid on Sunday too. The Passport to the Falls also offers discounted admissions at nearby attractions such as Artpark, Old Fort Niagara, and the Historical Wax Museum. Discounts at snack centers and gift shops are also included.
An adult Passport to the Falls is US$24.50. For a child 6-12 years old, the cost is US$17.50.