The Rambam in the Mishna Torah says that there are fifty one out of the 613 Mitzvot that fall under the category of עבודה זרה. He explains that idol worship first began when individuals began substituting Hashem's awesome creations for G-d Himself.
It began with worshipping the sun instead of viewing the sun as a proof of Hashem's power and might. Instead of the sun being the vehicle to intensify our belief in G-d, people got sidetracked and turned the vehicle into a deity.
The Rabbis tell us בכל דרכיו דעיהו, in all your ways, know Him. This means that we are to be aware of Hashem's Presence in everything we do. Even when we exercise, we don't make the exercise the ultimate, but it is a means of serving G-d better with a healthy, well tuned body.
Anything can become עבודה זרה if we give it too much importance and we take G-d out of the equation. Even Halacha can become עבודה זרה if we stop seeing at as a means of getting closer to Hashem and it becomes an end in itself.