We're going on a scenic autumn train ride - our DS (2.5) is seriously into trains right now. We're also going apple picking.
Corn mazes, hayrides, etc. are also fun, might be better closer to Halloween though.
For more at-home activities, we're always big on baking and cooking. Any apple-y, squash-y etc. recipes would work well I think. Making apple sauce, maybe? Or apple prints, where you cut them in half and dip them in paint and stamp them on paper?
Going on a nature walk to collect natural items of the season, or taking photos together and making a mabon viewbook of what your neighborhood/yard looks like in each season.
Looking forward to reading other ideas!