Non-disparagement, release scope, and the reference letter you can still negotiate after the offer is on the table.
A severance agreement is a contract, and almost every one we read has room in it. The five clauses that matter: the scope of the release, whether it covers claims you have not discovered yet; the non-disparagement clause, and whether it runs both ways; the reference term, which is nearly always negotiable and rarely asked for; the timing of the payment against your last day of coverage; and the confidentiality carve-out for your spouse and your lawyer.
You usually have twenty-one days. Use two of them.