That time of year is fine to visit. It will start getting a little cooler, maybe in the 50s F, Rain getting a little more frequent, but not most days. Beach will not be a concern.
There are a slew of museums, not only Amsterdam, but Leiden, Delft, The Hague, Rotterdam, and more, all easy travel distances, even for day trips. Most slant toward the arts, but a few nice maritime/ship museums, topics related to WW2 (Anne Frank, Dutch Resistance, Market Garden), History, etc.
You will be too early for Christmas Markets, those do not start until about the last week of November.
For events, I can't think of any major ones, but I usually search where I am going with something like: "Events for Amsterdam October" or Festivals. Usually that turns up lots of things, from small to large, and usually musical acts or festivals happening, but by then, Taylor Swift will be back in the US on tour.
As for me, my favorite activity is usually to sit with a beer on a canal, just watching the city move around me.