Letter of Separation
State timeline and intentions clearly. Â It does not need to be overly detailed, but it will go in your file with HR. Â There are plenty of example letters online for reference.
401k
Decide how to handle 401k, which Clark Howard would say: Roll it Over! Â Just need to call to see if roll-over is an option.
Benefits
Determine when health benefits from the old Company end and at the new Company begin. If there is any lapse (which is bad) between when your old coverage ends and the new begins, decide on paying for COBRA Insurance which may be offered from the old Company’s health plan.  Keep in mind, single-payer insurance can be expensive and COBRA insurance normally is not subsidized due to voluntary separation.  If the job-loss is involuntary, COBRA is actually subsidized and pretty economical.  Going on a spouse’s plan is an option as well.  Typically a job-loss is a qualifying condition for the spouse to update their benefits.
Stock options
If vested but not exercised, and it is beneficial to use them (after taxes, etc), don’t forget about them before leaving!
Vacation
If you have unused vacation and it has already been accrued for the year, ask about getting paid for it.
And lastly, enjoy the adventure 🙂