Glow bracelets/glow glasses for night time.
Fun shaped sunglasses-maybe statue of liberty. We went to Cancun and bought silly ones with sombrero & a mustache, we had so much fun with those! I think I got at Party City.
Personalized cup with a lid
We brought inflatable guitar, microphone, saxophone and had a karaoke party! Oriental Trading or Party City.
Maybe even dollar store? Cheap & easy to leave behind.
Disney cruisers do fish extenders-google fish extender gifts to get ideas.
Some of the gifts we snuck back to the room and left on the bed from whoever the cruise mascot is.
Carnival :It was Funship Freddy, now its Dr Suess
Any vacation that doesn't have a "mascot" we say its from the vacation fairy.