Not necessary, but fun are door decorations/magnets.
We always pack a pop up laundry bag from Dollar Tree. Its great for collecting laundry and for carrying it to the laundry room if needed.
My DD has food allergies, so we are all about sanitizing the room from top to bottom when we arrive.
I also pack nail polish because sand and pool water really wreck a mani/pedi.
Our passports always come along, also copies of our credit cards, medical cards, travel plans, etc.
We pack tumblers and my coffee mug...and lots of wine, water and mixers too. I mean, the kiddo needs her in room happy hour mocktail too!
The normal amount of medicine I think we might need and double it. Yup. After reading about a cruise where they ran out of a lot of different children's meds after a nasty bug swept through the kid's club, I now pack way more than we need.
For foreign ports, a phrase card with key phrases on one side and a little map on the other. Also, phone number(s) for the local cab company in case we ever end up in a pickle.
A collapsible tote/shopping bag for any extra "stuff" we end up needing to carry on the plane for the return flight home...a la, "we bought too many souvenirs!" I have used this tote on about 70% of our trips...in the airport when a bag or five was a pound or two heavy...just pull it out, load it up and carry it on. (Because its a shopping bag, it has never counted against my personal item or carry on item. Passengers are allowed to carry on all shopping items from the airport so its almost the same thing)
Last, I pack our Multi-Port USB Charger and our external charger (which is great for charging phones on the go!). We run our cell phone all night long (with white noise app going) and charging, we also charge the tablet, another cell phone, run a mini USB fan and charge the external charger...all with one outlet.
I work out of the country often, so I have packing down to a science.