* "Let's" is informal. It's a contraction of "let us," which is more appropriate in casual settings.
* The phrase is a bit vague. What exactly does "something formal" refer to?
Here are some more formal alternatives:
* "Let us proceed with a more formal approach today."
* "I propose we adopt a formal tone for our discussion today."
* "For the remainder of our meeting, let us adhere to formal protocols."
The best choice depends on the specific context.