Pop Century is going to have a Halloween Maze on the Generation Gap Bridge. I'm not sure how exciting it will be, but it's free. The aisle ways are supposed to be 5 feet wide, so it should be wide enough for a wheelchair.
Most resorts have some sort of activities on Halloween in the hotel lobby, but it changes from resort to resort. When I worked at the All-Stars, they had some coloring activities and candy decorating. Resorts also have a pumpking carving judging contest - each department carves a pumpkin and the guests vote on the most creative, funniest, scariest, etc...
Downtown Disney is also having Trick or Treating, but I'm not sure on which days. I'm sure someone on this board can post more information about this. DD is free, so if you feel it is too crowded, you can just leave without loosing the $30/ticket.