I would also suggest fridge water. And I was also going to suggest an extra special fancy cup but since you say you have tried cups...how about a water bottle? DS looooves drinking out of Mummy's water bottle and I have also bought him a Sigg water bottle with a puppy dog on it that he loves. I leave it full of water and just lying around and he will go and have a swig at odd times during the day.

Or a straw? My DS also loves drinking out of a straw. I have some Tupperware cups that come with a lid that has a straw hole, they are quite big cups, but that's okay. I have a packet of normal disposable straws and he is pretty much beside himself with anticipation when he sees me reach for a straw from the pantry. He will even go over to the pantry to point at the straws when he wants to drink out of one. You could even try an ordinary cup with a straw...perhaps you might capture her interest if you show her how to blow bubbles through the straw?