It's pretty easy to find things they can't do. They lack a level of abstraction that even small mammals have, which is why you see them constantly failing when it comes to things like spacial awareness.
The difficult part is creating an intelligence test that they score badly on. But that's more of an issue with treating intelligence tests as if they're representative of general intelligence.
It's like have difficulty finding a math problem that Wolfram Alpha would do poorly on. If a human was able to solve all of these problems as well as Wolfram Alpha, they would be considered a genius. But Wolfram Alpha being able to solve those questions doesn't show that it has general intelligence, and trying to come up with more and more complicated math problems to test it with doesn't help us answer that question either.