Just factory reset them. 

You can exchange them for credit with Amazon

Buy a club hammer. Smash them all to pieces. Incinerate the pieces at a high temperature. Hit them again. Once pulverised separate the ashes into four pieces. Put one one in garden waste, one in the bin. Bury the third deep in the earth. Scatter the final part to the four winds. 

If you fancy an upgrade Apple will do trade in on them.  I've also sold some of my previous phones to the various phone recycling companies you can find online.

Don't you do a factory reset as soon as you have a new phone?

Does resetting your iPhone to factory settings actually delete everything on the device, or is there anything that can be recovered?

These days, it doesn't actually delete anything, but nothing can be recovered. Let me explain.

Back many generations of iPhone, a factory reset would take an inordinate time while the phone literally wiped every byte of data from memory.

For the last several generations, everything on the iPhone is encrypted with a key that is generated randomly when the phone is set up. That key is stored only locally, on the iPhone, and used to decrypt data, on-demand, as you need it, but the original data in flash memory remains encrypted. When you do a factory reset, it wipes out that encryption key and marks iOS as needing initial setup.

With this approach, factory reset takes only moments, but nothing is accessible, since the encryption key is gone.

When the phone is set up again, a new, different key will be generated.

Anyone flogging an old phone or computer is reet menkle.