There is more than enough to do downtown. The Maritime Museum has a Titanic exhibit along with the ship Sackville.
Here is a document with lots of information.
You could rent a car and see some of the beautiful pastoral scenery, visit the north shore with its beaches and cliffs and then walk around Charlottetown.
It is a bit expensive and cheesy but a lot of fun. You get to see the view from the top of Citadel Hill, ride in the harbour and go on some main streets in town.
Generally the fall colours are best the second week of October. This year because of a spring drought and a hot summer the colours were a bit muted and they were at their peak more into the third week.