If you want to do something fun with it jailbreak it.

You can purposely mess up government surveillance by tampering with their collected logs. First, log into two devices using the same iCloud account. Then, leave your old iPhone, with your registered SIM, at a different location. After that, connect your other device to WiFi. Messages from your iPhone will be forwarded to this other device, even though you’re in a different location.

sell use as backup give to family friends if they aren’t bad

I usually sell them so I can recoup some money when I upgrade. I sell them in private.

You can resell them

iPhones retain their value much more compared to Androids

You donate it to your neighborhood Witcher.

an iphone 5/5s/5c would do a good music player.

Use it when your phone is stolen.

Sent from iPhone 7

Sell? Save one as a backup phone?