Lots of people do visit Disneyland alone, I think you would be fine. Plenty to do to keep you busy, entertainment, etc. Keep in mind Oct has Mickey's Halloween Party nights throughout that month on select days that would have Disneyland Park only where they are held closing early to regular guests, and you must pre-purchase a Party ticket to stay within that Park. Here is more info on this years Party nites and what dates they are on etc. Usually Party ticket will sell out quite far in advance, so you would want to get them about a month ahead of time at least.
And on the right side of the Anaheim forum page on the right side of the page under "top questions" will have alot of things to do and how to get there etc.
going alone...you can get around alot faster and get on rides quicker in the single line
are you staying at a nice hotel...i'd plan some time relaxing around the pool
google day bus tours - one i noticed on "viator" included a visit to laguna beach which is great and whale watching
Will you have a car? That can make a difference.
But either way, aside from Disney and Knott's, there are the Outlets at Orange for shopping - or Downtown Disney, for that matter. Bowers Museum in nearby Santa Ana www.bowers.org . Santa Ana Zoo (very small) is nice for a couple hours, if the weather is good (which it should be, in October) www.santaanazoo.org .