Just like the script from Magento 1.x, if you need to do some basic sandbox testing of functionality, you can do it within this simple sandbox script.
$bootstrap = \Magento\Framework\App\Bootstrap::create(BP, $_SERVER);
// any code you want here....
Now you have access to the entire Magento application.
If you want to work with classes, you can instantiate new classes from the object manager from within bootstrap.
$class = $bootstrap->getObjectManager()->create('Namespace\Module\Model\Test');
I know that when I need to tinker with a new platform, a bootstrap is always good to figure things out. Hopefully this will help you as well.