I traditional souvenir for travelers is a squished/smashed/elongated/pressed penny. I’m sure you’ve seen those machines where you put in a buck or two and a penny and the penny will be stretched and imprinted with a design. These days you generally don’t put pennies in (I think it is against the law to deface currency/coin), but there are usually copper tokens the size of pennies already in the machine.
I recently found on of these on the ground.
I’m not positive where it came from, but based on the image, it must have been a Niagara Parks location, and was most likely near Table Rock as the picture looks like the Journey Behind the Falls.
I did some brief searching for more information about this sort of collecting, and it is actually quite popular. The following are two sites of interest:
Squished Penny Museum – This site, although no longer updated, it has information on the history of squished pennies, along with stories from the owner’s travels.
PennyCollector.com – This site has a great listing of places around the world you can get these squished pennies. The Canada page shows that there are 15 places in Niagara Falls where you can get these. I’ve seen the machines at several of the locations mentioned.