If you have a passport/are interested in crossing the border, Bird Kingdom is on the Canadian side of the falls and is a really cool indoor walk-through aviary. Here is a link to their website (
Bird Kingdom). While in Canada, it's an easy walk over the Rainbow Bridge from the US side (when I visited Bird Kingdom, I parked near the aquarium and walked over the border, so it can easily be done on the same day as the aquarium). Personally, I wouldn't recommend the Aquarium of Niagara, it's very small, expensive, and there isn't much of note there, but if there is something you really want to see there/you're interested in hitting another facility, go ahead and visit.
Note that Buffalo Zoo is currently only open Friday-Sunday, and I don't know when that will change for the summer season so if you are planning to visit on a weekday check their website first to make sure they are open. Other than that, it's a nice, medium-sized zoo that can be seen in a couple of hours. I don't have any obvious tips for you, except for that if gorillas really interest you then sometime between 11 and 12 they are usually fed (the exact time sometimes varies), and that tends to be the best time to see them out and active. I think there's one or two feedings later in the day as well, but 11-12 is the one that I'm definitely aware of. As for other stuff in the cities, that really depends on what your interests are, but there're certainly some cool things to see, although no obvious "you need to see this!". If you're a bird person, then the Cave of the Winds at Niagara Falls has a large nesting colony of wild ring-billed gulls that can be a cool sight to see. Feel free to PM me if you want more info about the zoo and/or city in general, as I've been living in Buffalo for about six months now.