Blaspheming Islam/Mohammad (including doing anything inappropriate with a Quran), Pro-Israel commentary or symbols, openly promoting other religions, and Pride symbols would be the most obvious ones.
Eating, drinking, or smoking in front of fasting muslims isn't "offensive" imo, more viewed as insensitive. Depends on the person. Presuming it's like Malaysia, most restaurants shut down during the day during Ramadan (excluding Chinese restaurants).
If you're in a food court/hawker center with that has a mix of halal and non-halal stalls, there's two different sets of cups, plates, cutlery, etc. Putting non-halal plates, cutlery, cups, etc. into the halal area might get you some looks.