I think there might be a possibility of your dog getting hurt by a rambunctious young great dane, they often don't know their size or their strength. My vet owns a young one and Indie is only slightly smaller than her and at the dog park, she beat the crap out of him! She just didn't realize how big and strong she was. I think it would take alot of training and supervision to have them work out together.
OTOH, if you're looking for a dog to do therapy work with, I'd recommend going with a small dog again but find one with a great personality that's about 1-2 yrs old so you can see the personality that's already developed. I think it would be easier to do therapy work with a small dog that can sit on someone's lap and be petted and whose size wouldn't be intimidating or scary for the dog. Especially in nursing homes where the residents can be very fragile.
I've been looking into therapy work for Indie and alot of the sites I've looked at have lots of pics of small therapy dogs in people's laps, so that might be a better way to go.
As for the Dane, I'd google Great Dane rescue and try to get her in with them. They're familiar with the breed and can either take her in or help get her placed.
Good luck!
"I wonder what goes through his mind when he sees us peeing in his water bowl"
Penny Ward Moser
Save a life, make a shelter pet part of your family.