I do love Bedknobs & Broomsticks - glad to see so many list it! (But I own that on DVD and have watched it fairly recently.)
The first movie I actually watched on Disney+ was Alice in Wonderland, with my niece and nephew.
The first anything I watched were a couple of shorts, just in learning how to navigate the app,
and the first series - and my favorite thing on the platform so far - was The Mandalorian ....................................................(aka "The Baby Yoda Show!" )