The 3DS allows you to return to the home menu at any time during gameplay.
From there, you can use the web browser, make game notes, and do some other small tasks without interrupting the gameplay.
You can adjust the brightness whilst the gameplay has been suspended. HOWEVER, if you are playing a regular DS game, it cannot be suspended, it must be quit before you may adjust the screen brightness.
I wouldn't mind knowing myself if Super Mario 64 DS happens to turn off the screens. It didn't have brightness options, instead you could disable back-lighting.