660cc car adequate for getting around Kyushu?

Hi all! I am intending to travel around Kyushu (Oita, Kumamoto, Nagasaki, Saga, Miyazaki and Kagoshima) this October and car rental is on my cards. We do plan to visit Mount Aso, Mount Sakurajima, Mount Unzen and Mount Inasa along the way. Just wondering, would a 660cc car (Kei car) be sufficient to drive 2 pax in total (1 driver and 1 passenger only) and 2*20kg suitcases?

I think it mainly depends on the specific car you will be using. The power of the car will be no issue, assuming it is a moderately modern car. As for space and comfort, I think it will depend on the actual car model you will be using. I have used kei cars that feel more spacious than regular cars with a huge windshield and enough space for luggage. But I am sure there are other models that feel more cramped and don't have much luggage space.

It ought to get you where you are going, but you won't be speeding up any mountain slopes. Slow acceleration will be the most noticeable thing.
One problem though - are you aware that the area 1km from the Mt. Aso crater is still off limits? You can't get anywhere near it. As an alternative, you might consider driving up to Daikanbo which is close by. If the weather is clear, there is a great view over the whole area. And Nabegadaki is a nice waterfall just NW of Oguni town that you can walk behind. Mostly unknown and not crowded. The Kuju Flower Park is also very nice.
Takachiho is also nice to see and convenient when you are driving.
Within the cities, you might just park the car somewhere and walk to several nearby places.
In terms of history and varied points of interest, Nagasaki
is really the most interesting city on the island. It is worth a couple days to see the city, and another day to see areas nearby like Shimabara/Unzen or whatever interests you.
For Nagasaki:
https://www.youtube.com/playlist?list=PL7LcPcmoSg-gbw2yXNLK92siHyNqSNx...
the best sights are the Peace Park/Museum
https://www.youtube.com/watch?v=tFDCR5YLPOg
Glover Garden
https://www.youtube.com/watch?v=QdWRyHGW3rk
Dejima
https://www.youtube.com/watch?v=Qt3XF_Pqcx4
Koshibyo Shrine
https://www.youtube.com/watch?v=Mu_JnkVInTU
Gunkanjima would also be well worth seeing, but it takes 3 hours.

Kagoshima City has a number of great places to see, including Sakurajima:
https://www.youtube.com/watch?v=LxDmLuq6ssI
and Senganen Garden:
https://www.youtube.com/watch?v=lpnDK_Wux9Q
plus Shiroyama Park:
http://www.japan-guide.com/e/e4607.html
The ferry ride
https://www.youtube.com/watch?v=X7UYaxpN6yA
going over was also particularly relaxing.