"Nobody added salt to this soup, did they?"

Words with negative meanings like nobody, nothing, and never take an affirmative (positive) question tag. We use they to refer back to nobody.

"Well, I am still the head chef here, aren't I?"

The question tag for "I am" is irregularly formed as "aren't I?" (instead of "am I not?", which is highly formal and rarely used in spoken English).

"Let's just order a giant pizza instead, shall we?"

Suggestions starting with "Let's" always take the question tag "shall we?".