From Ohr Avigdor Shaar Habitachon by Rabbi Avigdor Miller
What kind of "service to Hashem" is Chovos Halevavos referring to? He’s not talking about a person who says he won’t put on tefillin until he has enough money. Nor is it a person who says he won’t keep Shabbas. We’re talking about a person who keeps everything anyhow. Refusing to keep the mitzvos is out of the question. The author of Chovos Halevavos has in mind a lifetime career of service to Hashem
The minimum service of Hashem is learning to thank Hashem for Tverything
In addition to our daily program of keeping the mitzvos of the Torah, which every man and woman is expected to keep, he expects you to devote yourself to His service, a certain kind of endeavor called avodas (service of) Hashem. Chovos Halevavos expects you to undertake that endeavor without any advance promises or guarantees. We are discussing a kind of avodas Hashem that a person has said he’s willing to undertake on condition that Hashem puts down a pledge or deposit in advance, but Chovos Halevavos is ridiculing him and saying, “How can you ask for a pledge before you start doing avodas Hashem?”
So, what is avodas Hashem? Now if you look back on what we have learned in Chovos Halevavos and ask, “What’s the minimum of avodas Hashem, less than which nobody can feel he has fulfilled his duty?” The answer that will jump at you from these pages is that it’s a matter of learning to thank Hashem for everything.
He’s not talking about learning Torah, as essential as that is. He’s not talking about going out and teaching people to live a good life and observe the Torah. All these are good things. But first and foremost, avodas Hashem is the subject of studying all the wonderful things of This World and being grateful to Hashem for them. This is something that’s so far from our minds, it doesn’t enter our thoughts that that’s avodas Hashem. Yet that’s what he’s talking about. That’s the minimum avodas Hashem.