What's a good place to visit for 3 days? vs Where's a good place to visit for 3 days?

Either one is correct. Native speakers might be more likely to say "what's," but i can't even say that with certainty. Use whichever locution you prefer.

You might choose "what" over "where" when the desired response is for a type of place rather than a geographic location.

For example, "a museum" would be a different kind of response than "Cancun."

Edit: This response was intended to be @Marsce.

Thank you. I appreciate the help.

Ok, thank you. Big help.

Either Cancun or a museum is a reasonable response to either form of the question.