Simple:
* "I'm just minding my business."
* "He should be minding his own business."
Adding Context:
* "I'm minding my business, but I can't help but notice..."
* "I'll be minding my business until you decide to apologize."
* "She's too busy minding her business to notice what's happening around her."
Humorous:
* "Minding my business is a full-time job, thank you very much!"
* "I'm a professional business-minder, and I take my duties seriously."
Remember: The tone of the sentence will depend on the context and your intention. You can use "minding my business" to express a neutral observation, a lighthearted remark, or even a hint of annoyance.