Another vote for Bali ^
You can always spend about $40 to rent a car, include a driver, to show you around Bali.
Everything is cheap and it's safe to travel alone.
Be careful of those people (tourists or local) that you met during your trip. If they do drugs, STAY AWAY, as Indonesia don't take drug crimes lightly. Same goes for Singapore.
With that out of the way, you'll have a fantastic time there
