There are plenty of alternative ways to express the phrase "it's just a thought." One can say "it's simply an idea," "it's merely a notion," or "it's nothing more than a suggestion." Other synonyms include "it's only a passing fancy," "it's a fleeting concept," or "it's a passing notion." Some people might also say "it's just a brainwave," "it's a random idea," or "it's an idle thought." Ultimately, the choice of synonym will depend on the context and tone of the conversation. Regardless of the choice, all of these phrases convey the same general idea: a thought without weight or significance.