Working with what I've read about balls from Jane Austen's regency era novels... you'd want some chairs and couches around the edges to give people who are watching something to lounge on and possibly a table or two to hold refreshments. On the walls... banners, maybe? Large paintings would work, too, but don't think too many of those are available. Instruments on the "stage" might give a good hint if we had any available yet, not sure. Something piano-ish would be perfect. Anything resembling a chandelier would give it a nice touch, too.