Severance offer, what should I ask for? - what worked for you?

My manager said the package is final, but the letter gives me time to review. I have read a lot of threads here and could not find my exact situation. There is a non-disparagement clause. Did anyone negotiate and succeed?

Thanks for writing this out. I went through something close to it 4 months ago, and it settled down once I kept a simple log of every call and form.

I asked for two more weeks and a neutral reference. They agreed to the reference and added a week.