What activities would you like to do in this space?

Many people use a former formal dining room as a library. Bookshelves lining the walls, comfortable chairs with excellent reading lamps, and you have pretty much finished with the decorating.

If you have children, you might consider putting a "library" table in the room, and be sure to provide a comfortable chair for each child's body size. This would make it easy to supervise the children while they do their homework. (I would also make the lower shelves of all of the bookcases into cabinets to hold the children's supplies.)

You could use the room for a grand piano and use it for a music room.

You could paint the walls in a similar manner to an art gallery, hang works of art or display pieces of sculpture, and install special lighting to highlight these pieces.