I wonder if there are angels for the tree topper that have disabilities? Wheelchair, walker, white cane......... it kind of depends on the disabilities you are representing.
If you can, I think it would be nice to depict able bodied children playing with disabled children. Just a little reminder for folks to look past the disability.
I see your ideas for representing many recreational activities that can be enjoyed from a wheelchair. How about showing a job that can be done from a wheelchair. For example, if you show a bunch of people skiing or sledding, maybe there could be a hot chocolate stand ran by a wheelchair bound person.
OK enough of my random thoughts. I'm sure you will get many great ideas and do a wonderful display. Don't forget we like pictures!