Open Jaw Flight - start with Surabaya or Yogyakarta
 

Flight options from SIN (Sunday-Sunday):

1. fly in to Surabaya (direct), land 17:50; fly out of Yogyakarta (stop in Jarkarta) take-off 13:40
2. fly in to Yogyakarta (stop in Jarkarta), land 16:10; fly out of Surabaya (direct) take-off 18:40

There will be 6 full days, not counting in/out bound days. Plan is to start in Surabaya, mainly for Mount Brormo, then make our way to Yogyakarta somehow for Borobudur and Prambanan Temples. Secondary interests would be scenic rice terraces, calm warm water snorkeling (preferably towards the end of the week).

Trying to decide should we start in SUB or flip the trip around and start in YIA. Between Surabaya and Yogyakarta, which one will be less boring or less likely to run out of things to do?

My general approach would be to first go to the destination where I would be more active and then to the destination that is easier. I don't know those places, but if one involved climbing a mountain and the other was snorkeling/temple walking then I would probably go up the mountain first.

It would be like you wrote, to do the easy stuff "preferably towards the end of the week".

IDK why you mentioned flight times, given that your 6 days don't count the travel days. Sounds like you are looking at an open jaw from somewhere and going via SIN to the two ends of the open jaw. Ok, I would also look into a roundtrip to SIN and one-ways between there and the two places. But, you really need to figure out the travel between the two places.