Fill the room with ergonomic furniture that doesn't distract from the piano or create poor acoustics. Guests should be able to view the pianist while enjoying the ambiance of the room. Create a space wherein both listeners and musicians are inspired to relax and enjoy each others' company!
A piano room should be warm and inviting for those who wish to play and those who wish to listen. Cloth couches and chairs will help with sound dampening and provide a place to sit and listen. Remember to also include a storage area such as book shelves or cabinets.
Furniture for a piano room should include a piano, of course, as well as additional seating. There should be comfortable straight-backed chairs for the listeners. There should also be a bookcase to hold sheet music and music books. Please include a lamp near the piano to spotlight the music.
First of all, in order to call it a "piano room", you will probably need a piano. Then, perhaps some sort of seating around the edges of the room for anyone who may want to listen to someone playing. Some plants might be nice to make you feel like you are outside.
If it is a small room, the piano and the bench should do it. Have a music stand available too, and an extra chair or two for other musicians that may join the pianist.