Kumamoto Station is located in the Eastern part of Kumamoto, but affords easy access to the Aso area by local train, to Amakusa by bus, and to the city center by bus and streetcar.
There is a one-day streetcar and bus pass available for 500 yen, and two days for 800 yen. This is useful especially if one wants to visit several locations in one day. Suizenji Park, Kumamoto Castle and the Prefectual library and Literature center are on the streetcar line, and Kumamoto University, etc. can be reached by city bus.
A few words of warning however: the streetcar is intuitive and well-marked, but the buses need negotiating -- information is only in Japanese. Also, if you plan to go to Kumamoto Castle and downtown areas only you will not need a pass. A one-way ride on the streetcar is 150 yen and aftr that it is all on foot.