A few months back, I tried to book into the Kosa and Charoen Thani next to the Sofitel as they certainly have no problem for guests, but they were full...so I motored around looking for other hotels and came across The Khon Kaen Hotel. It's only about Bht600/night and is good enough and never full and not a problem with guests either. There are a few small resort-type restaurant/bar places around the hotel where you can have a better breakfast than the hotel offers. This hotel is on the main drag Klangmuang Road that is a through route down to the lake near The Beach and has the Fairy Plaza also. The plaza infront of the Sofitel is not so exciting. Fairy is better and has Khon Kaen's only Pizza Company restaurant.
http://www.khonkaen.com has a lot of useful information and maps.