The sex toy(vibrator) is indeed a better idea! pulled this of with a female friend of mine... Put some make up in the bag as well! She reacted irritated at first ,"like my parents are here" "what an inappropriate gift" bla bla.. I just smiled and didn't even react. Later she was like: you prankster , what did the girl say when you bought make-up and a sex toy at the counter?. (Hitted on her as well haha.. The counter girl blushed, and I said to her:"I'm having a private party " and winked) so I told this to the birthday girl.. And all the other guest where laughing including her parents. End of the evening we were almost alone ,as she confessed she never owned a vibrator.. I said believe me you don't know what your missing! She asked me how it worked and I showed it to her
Key to this : Be open minded!, and don't give a shit what the guests are thinking!
Good luck