Depends on if you want to define a necessity more broadly or more narrowly.
Narrowly almost everything is unnecessary as Johny states above.
More broadly, I'm with VT Speed, anything that you determine you need for your tour is a necessity. Multi-tool gives me peace of mind, even though I may not even use it (hopefully). Necessity. Camera is a necessity, because I am not going to tour and see all these neat things without recording the images to show people later. Laptop, for under a week tour I can do without and just write a journal when I get home. But if your laptop keeps you from going crazy while you have insomnia, I'd classify that as a necessity.