My dad let me have his laptop 3-ish years ago. It is a Dell lower to mid-range referbished. Not worth a lot but I'd like to clear it out and offer it to someone -- still with Windows installed and legal.

How do I go about doing that?

It should have a windows key on the case of the laptop somewhere

Most of the time these days it's embedded in the UEFI firmware (todays version of BIOS). If that's the case, it'll reactivate automatically after a clean install, as long as you install the same version (Home, Pro, etc.,)

https://answers.microsoft.com/en-us/windows/forum/all/how-to-recover-your-windows-product-key/8687ef5d-4d32-41fc-9310-158f8e5f02e3

You can do a factory reset; Start Menu > Settings > Updates & Security > Reset this PC. Or you can use The Windows Media Creation Tool to make a bootable USB to do a clean install of Windows.

And as the poster above said, you don't need to Windows key anymore to do a reinstall.

So I can run one of the nuke CDs on there and then do a clean install? I haven't used Windows in years but not sure I trust just a Windows reset to clear my dad's info. He had some tax info on there.

DBAN. Then fresh install.

The built in reset should be fine against everything but targeted data recovery attempts, and may be fine against that. If you want real belt-and-suspenders level peace of mind, pay $11 for PartedMagic and do an NVMe Secure Erase, ATA Secure Erase, or NWipe (DBAN equivalent), in that order of preference.

The first two implement full disk erases through code in the drive's firmware, for NVMe SSDs or SATA SSDs/HDDs respectively. The 3rd is the old fashioned "write a bunch of 0s, 1s, and random data over the whole disk over and over again until the original data is undetectable". It's not as effective on SSDs due to wear leveling implemented in the drive controller, it's theoretically possible for original data to go unmodified if the wear algorithm favors other locations on the drive.

Thanks for all the help. After reconsidering my options, I am in the process of installing Ubuntu on it and will set it aside to run as a web server. I have some self hosted projects I want to use it for. Not sure why I didn't consider this before.

Server with a built-in UPS. Nice!