I have never stayed in a hotel in Plovdiv in my life as I have rarely stayed over a day and when I have, one of my friends there has invited me to spend the night at his, or her home. On the picture there is Novotel Plovdiv, which I have heard good stuff about. It's on the north side of the river Maritza, not far from downtown. A great, but relatively expensive hotel is Trimontium, located on the Central Square. Other than that I am sure there are plenty of privately owned small hotels and hostels, but I don't have any personal information about any in particular.
I have been set there by the company that asked my counsel in Bulgaria, so I may not complain. Honestly: I would have preferred a smaller hotel, more intimate. Anyway: a real 4 stars hotel, rooms, facilities, accessories, everything of high quality standard. The restaurant is good, too. No wi-fi internet connection.
Swimming pool, fitness centre, night club, sauna: I didn't try any of them, but I heard they are quite ok.