Most hotels now use keycards to unlock room doors (instead of keys). Some hotels suggest you return the keycard after your stay, or leave it in your hotel room.
However, you don’t have to do either of these things. Hotel keycards are deactivated as soon as your stay is complete, and you are free to take them if you wish.
Keycards can provide a fun memory of the hotels you’ve stayed in, and you can even show off your growing pile to admiring friends and family members.