Some of what you'd expect, there were Jewish and Christian communities there. The Christian community is actually kind of interesting b/c they were a Gnostic sect that believed there was kind of a heist to move Jesus out of his tomb and that's the version of Christianity that Muhammad was familiar with so it shows up in hadith a lot. There were Zoroastrians. These are the groups usually described as dhimmi.

There were also pre Abrahamic religions, like the Yazidi we heard about with ISIS. There were some of the cults from Roman times still hanging out,. And besides Zoroastrianism, there were some other religious groups from Persia, like the Manichean. There was even some Buddhism.

But there was a lot of idol worship. It's stuff that was varied, but would include household/tribal gods, ancestor worship, and animism. Before Islam, the Kabba was actually surrounded by idols from all these different groups and was already an area of regional worship.

Basically there was a lot of stuff, as you would expect from an area that served as kind of a crossroads of the ancient world. They got beliefs from the east and west going through. Some beliefs were adopted, some had parts taken and amalgamated into new religions or combined with older religious traditions.

Karen Armstrong is a popular writer and has a short history of Islam that you might like.