I tried to come back the next week, which was a stupid thing to do.

How could she even have thought of doing such a stupid thing?

If one of them found the stupid things on his own, fine.

He'd done a lot of stupid things in his life.

"Why did they give themselves away by doing such a stupid thing?"

You did a stupid thing and your face is red.

Those that have were just doing the same stupid things.

It might even have been a stupid thing to do.

The stupid thing for me to do would have been to follow her.

It would look like too stupid a thing to do.