15 days in Java itinerary- domestic flights

If you're going to Ujong Kulon and or Krakatau on your own that's quite expensive IMO.
Yogya's temples, Bromo & Ijen solo is reasonably easy.

Ujung Kulon is cheap to reach by public bus to Tamanjaya.
However, Krakatau is an expensive boat charter trip, and is organized infrom Java from the beach resort town of Carita, not from the mainland part of the park.
Both are way off the route to your other planned destinations, and wouldn't easily fit into 15 days with those. If Krakatau isn't a must, skip both, and visit one of the East Javan national parks instead for forest and wildlife.

Any suggestion for a better alternative? I'm not sure which park would be the most interesting.

For rainforest and turtles, Meru Betiri.
For most wildlife, Alas Purwo.
For easy access, decent accommodation and still good wildlife in drier habitat, Baluran.

If you'll be in Banyuwangi area for a while (Ijen, Meru Betiri, Alas Purwo, baluran etc) you might also want to consider flying in to Bali. By land journey it's just slightly closer than Surabaya.

If you're already putting Bromo it will only add 1 full day to your trip if you're that short of time (for just ijen only). But you'll have 2 sleepless nights in a row, 1 due to sunrise at bromo and another due to all-nighter in Ijen.

You can do your hike to the Bromo crater in the afternoon before for 10% of the crowd. After done with the Bromo stuff from 9ish morning you can find the shared van to Probolinggo. Train Probolinggo to Banyuwangi (4.5 hrs) at 11 (risky) or 4pm ish.

I arranged for a seat at a shared car to Ijen with the guesthouse and get a little sleep. Departure is close to midnight. Probably you'll be back at your accomodation by 10-11. Plenty of time to catch the 1pm train going west (surabaya or wherever)

Of course there are more frequent buses instead of trains but their travel duration are uncertain. If you manage to team up with other people, will be worth it to get a private car.

Thanks for all your suggestions. I just realized there is an airport in Banyuwangi, so I might fly there instead of Surabaya and save some time. If you have any more ideas, I'm open to suggestions :-)

Yes there's an airport in Banyuwangi, a nice little one with flights only from Jakarta and Surabaya (3x each day).
As consideration for comparison, Train from Surabaya is 7.5 hours and bus from south Bali probably 5.5 hours or so

Hi.. if i see from your itinerary you can fly into surabaya first so you dont waste much time. After arrive in surabaya you can go to Malang city, there is beautiful city which is surrounded by many mountains. You can see many waterfalls, stay at night in tree house, beautiful beaches which is still virgin (i said virgin cause to go there u need a guide and u must pass forest first with walking, vehicle is not allowed), you can go to bromo mt, semeru mt, etc and enjoy many beatiful landscapes. O yeah i almost forgot, if u want to go to bromo mt or semeru mt you must find privat car, u need travel agent service, because there is no public transportation to reach that place.
After you finish in Malang, you can go to yogyakarta to enjoy many temples like borobudur and prambanan, you can go to yogya palace, and merapi mt. If you want to buy many kind of handy craft and Indonesian culinary you can go to Malioboro street.
After finish in Yogyakarta you can go to ujung kulon and visit krakatau and the forest. If you still have time, you can go to Jakarta city. But jakarta is crowded city, i'm not sure that you can enjoy that city. After finish in jakarta you can fly back to singapore again.
Ok, thats my opinion, if still curious about many beautiful place in Java you can ask me. Have a nice trip...