You need to know there are 2 sides in 箱根小涌園, for one side, it's like water world where you can wear your swim suit. I think this one is more suitable for children. There's another side where you cannot wear anything. the feel there is extremly if you go at around 5-6pm in the evening. Very relaxing.
There are 3 different accomodations for 箱根小涌園 actually. I have stayed in their hotel before, a very decent one and not cheap. a very spacious room with a very large garden and its own 溫泉. the one that you may have stayed is their B&B one, with the most simpliest facilities and hence it's the most cheapest among the three.