The phrase "like fun" is often used to describe something that is unlikely or untrue. However, there are several other synonyms that can be used in place of this phrase. One option is "as if," which is often used to express disbelief or skepticism. Another option is "yeah, right," which is used to convey a sense of sarcasm or irony. "No way" is also a common synonym, particularly when reacting to an outlandish suggestion or claim. Additionally, "that's a joke" or "you must be kidding" can be used to indicate that something is not to be taken seriously. Overall, there are several phrases that can be used to convey the same sense of disbelief or skepticism as "like fun".