More and more phones can read NFC tags nowadays.
Samsung and Sony are selling (Smart) Tags and NFC stickers. The tags are quite a lot more expensive than the stickers. You can print this NFC keyfob and put the sticker inside.
This video gives some background into NFC and NFC smart tags:
I created a customizer script so you can define the size and shape of the tag for your stickers. Details at thingiverse:
I used these stickers: