I think
rain ponchos are a must. You can get very small ones from some of the "Dollar" stores.
Depending on the time of year, I carry a
small fleece blanket for my parents. We go often in December and by the time Illuminations starts it is getting chilly.
We normally wear
wind breaker type jackets on the plane but again this is dependent on the time of year.
In the parks I carry a Britta
filter water bottle. I used to carry it on the plane with me full of water but the last time I flew I had to pack it empty in my checked luggage. I don't know but I suspect you could bring it through security if it was open and empty. By March for you, things in airport security may change again.
I also carry a
sting-kill tube because DD is allergic to bees. I carry a small chap stick along with two small sunscreens, one for the body and another to use on faces (it does not sting when it gets in your eyes).
I also usually throw in a few pieces of
hard candy to tide us over before a meal or if the last meal was very garlicy and we need a better after taste in our mouths.
Our camera takes rechargable
batteries so I always carry the extra set of them with me.
I take
tissues because the sight of the castle always tears me up!
I wear a passport holder around my neck in the parks to hold my tickets and fast passes.