When viewing a single post, get_the_ID(); used within the loop will return the current post’s ID.
But I don’t have a reference page that has the list of $query_vars params…
Dumping it
global $wp_query;
var_dump($wp_query->query_vars);
would provide such a reference.
Where you’d subsequently see that $wp_query->query_vars['page_id'] yields the page ID.