North Bay has a nice waterfront that has both a Mini Train to ride and two Carousells. Threre is most always somting to do at our waterfront. I am not sure if they will be open in October they are both more of a summertime thing but if they are open your 4 year old will certainly enjoy them. Just outside of North Bay(about 2 km) on Hwy 17 heading west towards Sudbury there is Duscheney Falls. There is a parking area off the highway so you can not miss them. Great trails and there is always people there so you will feel safe. The fall colors are at there peek right now but there should still be lots of fall colours if you are only a couple of weeks from now.
There is also the Chief Commanda II. This is cruise that will bring you around Lake Nippising they have several different tours available. If you Google it, I am sure you can find more information.