on Princess its a soda sticker that goes on your sign and sail card,"technically" you are not supposed to share.
If you have teenagers i would buy it for them only. I do not buy it for the young ones.
There are tables all over the ship on the first day or two where you can purchase your soda sticker, you don't have to buy it on the first day, you can buy it any time from any bartender. You also DO NOT have to buy the special insulated souvenior cup if you choose not to,you can buy just the sticker and save yourself a couple of bucks.