Upvote:2
You might be able to check in your luggage only up to Singapore on the way back. And since I assume you're Indonesian, you don't need a visa for Singapore. So picking up your luggage and going out wouldn't be an issue.
However, depending on the pricing tier of your ticket you might be able to change it -- or not. And even if you can change the last leg, Singapore - Surabaya, you'll probably have to pay something. But it will probably be cheaper than buying a new ticket.
If you don't get on the last leg without talking with the airline (and if it's a cheap ticket even if you do), the ticket will be cancelled and you'll need to buy a new ticket.