If you are moving from Kumamoto to Beppu, or stopping on the way in Aso or Oita, you might want to consider riding the Aso Boy train.
The train usual run the weekend and holiday otherwise the scheduled route is done by the Kyushu Odan Tokkyu train.
The Aso Boy is themed around the mascot Kuro, a black dog that you will find across the train.
There is four car in he train. I would recommend trying to get a seat in car 1, row 1 to 3 in Beppu direction or seat in car 4, in the las row (8 to 10?) in Kumamoto direction to have a view from the big front window. If you reserve the seat at the other end, you will ride the train backward and just get a view from the back of the train, that is also not bad at all. Your second best option is car 3, that is the most themed car, with special seats as well as the counter where you can buy food and souvenir and the kid playground. Well, the kid playground might also be a reason to not take a seat in car 3.
Going back on the subject of food, they offer different snacks such as pudding as well as different drinks. They also sell bento box that you have to reserve on the official website once you have your ticket, you need to reserve the bento around a week in advance minimum.
As I was a bit late on booking the bento, I could not get one, but there was plenty of options at the Aso michinoeki (roadside rest area) located next to Aso station. Most of them feature red beef (akaushi), or horse meat. You might want to memorize the kanji for red beef 赤牛and horse 馬 if you do not want to make a mistake in your choice. I also had a cup of sake that I previously purchased earlier at a local brewery in Takamori. In the train I grabbed some yuzu flavored cake and a rum caramel flavored pudding (as the regular caramel pudding was sold out). You can also purchase different Kuro themed souvenirs.
As there is only a small number of limited express daily running on that line, if you happen to use it during, why not check to take the Aso Boy?
If you want to ride it, just make sure to make reservation as the train is reserved seat only.