What kind of sentence is might be a good idea

"Might be a good idea" is not a complete sentence. It's a phrase that expresses a suggestion or possibility.

To make it a complete sentence, you need to add a subject and a verb:

Examples:

* It might be a good idea to call a plumber.

* Taking a break might be a good idea.

* You might be a good idea to reconsider your options.

You can also use it as a dependent clause in a larger sentence:

Examples:

* Because it's getting late, it might be a good idea to go home.

* Since it's raining heavily, staying inside might be a good idea.