Ask for more money and benefits. Remember that their first offer isn't necessarily the best. Don't underestimate the power of guilt to squeeze a bigger severance package out of your former employer. Your former company might be willing to sweeten the pot, but you have to ask. Get a good reference before you leave.
Most people think of a severance package in terms of cash and benefits.
But, unless you're about to retire, don't forget about a letter of reference. Rehearse your referrals. Call everyone you plan to use as a reference and talk with them about what they'll say. Don't just ask them to sing your praises. Tell them you'd like to discuss your strengths and weaknesses.