Be careful of this...I did originally look at Sumatra but because the visas are on 30 days for Indonesia I wanted to make sure I reached Bali with plenty of time to sort out the gear and make arrangements for flying to Australia. It’s a shame so many countries have such short visas.
"Proof of onward travel
Immigration officials in Indonesia may ask you for proof of onward travel (eg, a return or onward air ticket). You should make all reservations before leaving for Indonesia. Some airlines have refused to board passengers without evidence of onward travel."
https://www.gov.uk/foreign-travel-advice/indonesia/entry-requirements
Actually, that reminds me, I have a feeling the same thing might apply to a 30-day visa-free entry to Thailand too.