Like many towns along the Rhine, it has it's own castle. The castle is not very big and part of it seems to be used for weddings and other gatherings. There was a wedding going on there when we visited this past weekend. There is also a tourist information center inside.
The best part of Eltville was the castle gardens. The gardens had the most beautiful flowers I've ever seen. We spent quite awhile wandering the gardens and looking out over the Rhine. The weather was perfect for it.
So if you're in the area, stop by Eltville and check out all this little town has to offer.