Japan has much to offer:
Technologically advanced cities.
Advanced transport and infrastructure, including urban rail and high-speed trains.
History: buildings and museums, well-preserved or rebuilt.
A unique culture.
Thailand has great food, value, friendly people.
Mae Hong Son has wondrous mountain scenery, stay at Panahuts.
Bangkok has big city attractions with culture, see Bangkok.
General advice on Thailand
Seoul is an advanced and sophisticated city.
Seoul has an excellent subway system with many lines.
T-money cards are most convenient; they are reloadable and fares are discounted.
Trips are 1,050 Won for up to 10 km, so location is not critical, but it is good to stay close to at least two lines.
Visit Seoul is an excellent guide, with details on every possible tourist attraction.