Mummies or tombs interest us most, but a temple or a pyramid is fascinating too.
Both the words mummies and tombs are plural and could be replaced with they, so we want to use interest. We would only use interests in third person singular (he, she, it).
Now the next part is more tricky. Even though the intended meaning is the same if we say, "a temple and a pyramid are fascinating." We need to use "is" because both objects are singular and are separated by "or rather than "and."
It's the same idea as the following sentence: "A dog or a cat is a good pet."
