When it's time to "get serious," there are plenty of synonyms to convey your message clearly. If you want to emphasize the importance of a task or situation, you might say "get down to business" or "get to work." Alternatively, you can use more forceful language, such as "buckle down" or "shape up." If you want to sound more ominous or threatening, you could use phrases like "crack down" or "turn up the heat." Ultimately, no matter what synonym you choose, the goal is the same: to communicate the idea that it's time to be focused, diligent, and committed.