stkr.link

NFC technology is widely used for many different purposes. Unfortunately, this means that there are many choices and little explanation of what they mean.

If you want a quick and easy answer that works for most people, get NTAG215 stickers, like this set or this set.

Note, if you want to place these NFC stickers on metal surfaces, you will most likely need to use "anti-metal" NFC stickers instead, like this set or this set, because they have a ferrite layer that lets NFC work correctly around metal.

Many other options are out there, but NTAG215 should work in most situations, and these stickers are fairly easy to find. They have 504 bytes of writable memory. For comparison, NTAG213 supports 144 bytes, while NTAG216 supports 888 bytes. Other tag types exist as well, such as MIFARE Classic and Topaz 512, but the NTAG series is currently the most popular and well-supported for general purpose information storage.

If someone finds a blank NFC sticker, their first reaction might not be to open up their phone's NFC app and scan it. So, you can stamp the sticker or write something useful on it, like "Property of John Smith / stkr.link". Please avoid ballpoint pens, because they can damage NFC electronics if you use a lot of force.

A custom stamp with writing or image of your choice can be surprisingly affordable. Since the stickers are small, the stamp should be even smaller. For example, here is a self-inking 3/4" one.
You can use the stkr.link logo (logo hereby released under CC BY-SA) or anything else you like. Linking to this website might help people to understand what to do if they find such an NFC sticker, but that's entirely optional.

If you don't want to use a stamp, then a permanent marker should work well. Be sure to use one with a fine enough point that you can legibly write your message.

You can program a stkr.link to show a message, open a URL, initiate a phone call, write a text message, begin creating an email, and many other things. But don't overcomplicate matters. There is limited space available (see above), and besides, you should make it easy for someone to contact you if they've found something of yours.
For example, you can make your stkr.link initiate a text message to your phone number with the pre-written message "Hi John, I found something that might belong to you".

To do this, you'll need to use an NFC writing app. On Android, NFC Tools works well. Alternatively, NXP's TagWriter is available for both Android and iOS.

If you want to prevent your stkr.link from being edited later, NTAG stickers support "locking", or setting a read-only flag. You can do this using the NFC writing app of your choice. Note, you will not be able to make any further changes to the stkr.link afterward.