The first weeks at a new job are busy, and benefits paperwork is easy to push off. A few of those forms have hard deadlines.
Health insurance
New-hire enrollment windows are often just 30 days. Miss it and you may wait until open enrollment, months away.
The I-9 and W-4
Your employer must verify your I-9 documents within 3 business days of your start. Bring your ID on day one. Fill out your W-4 carefully so your first paycheck isn't a surprise.
Your old 401(k)
There's no rush here, but don't forget it. You can leave it, roll it into your new plan, or move it to an IRA.