I think a nice large plant in a fancy planter is the best thing to have in a bay window but if you think this could obscure too much light then a small half moon table with an unusual antique or ornament placed on it.
If your bay window is in a living or family room, it is just begging for two comfortable easy chairs and a small table and lamp. This will make a cozy sitting and conversation area away from the rest of the seating. You can go with wing back chairs for a more formal look if you desire.
I would recommend a window seat, and decorate the same with large throw pillows, to make a great place to sit, I would recommend a build in. If you want a moveable piece of furniture I would use a lage bench and make the same effect with cushions and pillows.
You will want to consider how much light comes in through your bay widow. If it is in a sunny location you will not want fabric covered furniture, or it will fade. Chose metal or wood instead.