Scooter - DS had a micro - 3 wheels at the front easy to ride and heaps of fun (he has dyspraxia)
Horseriding - amazing for confidence, but think she would probably be better in private lessons where the instructor can just focus on her - this is how DS started - and I know this can be expensive. What about riding for disabled - I know she is not disabled technically but maybe you could ask them?
If you can stretch to riding lessons - where DS and I ride offer 1/2 private, it would give her the start and then she could move into a group? At DS RS they do walk trot classes for beginners and the beginners always have someon leading them for trot - so the kids can get a "feel" of trot without having to control the pony. Also ask at school if the have any experience of SN. At DS RS they have a group that are all children with some form of SN, and DS riding instructor has dyslexia, dyscalcula and mild dyspraxia - she can remember the frustration as a child. Incidentally she is also now a top competitive rider, representing her county. (Dressage and showing, not jumping). You have to be fit to ride, and is so good for core strength, so fab for sitting. re-reading your original post I would say 1/2 an hour to start with would be best - I think an hour would exhaust her, and you want it to be fun. DS started out with 1/2 hour lessons.
Martial art - this can be really reasonable, but the key is the right Club. Ds did Tae Kwon Do in Dubai and has just started karate here. I couldn't find a TKD club with the right attitude, but have found a Karate club just down the road, and the master is amazing - fantastic with kids. Lots of groups run mix age groups, so her ability wouldn't necessary stand out. Focus is very much on oneself....also because martial arts are often recommended for children with co-ordination issues, there seems to be a higher proportion there!
I think the key is finding things where their successes (not matter how small) are celebrated, and they are not asked to compete against other kids, but just do there best.
Bike - 18 months ago ds would not even try to ride a bike, he now goes mountian biking with DH, it took a while and alot of effort on our part to get him there, but he loves it. So just don't rule it out.
Oh and we just got an Xbox kinnect - fab, the energy he uses, but man he has to concentrate to co-ordinate.