Olaf, you could check this link:
October 2019 Events in Tokyo | Tokyo Cheapo
It isn't everything, but it does cover flea markets and some events.
The leaves won't be changing colour in October.
It's still about 28 degrees in the day time here, and around 20 degrees at night.
Late November is usually best.
Just walking around Tokyo is fascinating in itself.
Go to the Metropolitan Government Building in Shinjuku if it's a clear day and enjoy the view.
Agree with Squirrel that a wide angle lens is very useful.
First-time visits to the places mentioned are great.
There are a lot of visitors here at the moment for the rugby world cup.