Can I use a Singapore visa to go to Indonesia and then return to Singapore?

Yes, but when you enter or leave Singapore, you should give your passport to the Singapore Immigration and Customs Office for inspection.

May I see your tourist visa? It is estimated to be valid for 3 months. It says multiple entry, which means you can enter and leave Singapore many times in three months. However, it is generally stipulated that tourist visas cannot exceed 15 days at a time in Singapore. (It's none of your business)

For example, I go to Singapore, then go to Malaysia, then go back to Singapore, fly to Hong Kong and then go back to the mainland.