There are two other artifacts in that level besides the Idol, a relic (plate) and a mask.
For the locations of all the artifacts, I'd recommend the walkthru at GameFAQs.
It has them all in a ordered list at the bottom of the document. (They are also mentioned in the walkthrough itself. Spoilers of course.)
BTW, on the pages that show the levels in the "Saved Games" book, the artifacts are all shown. So you can usually narrow it down to the sub-level easily. Most of the artifacts can be found by simply exploring the levels thoroughly, but there are a few tricky ones.