While the rules on attire may have relaxed for Westerners, there are still rules on what women can and cannot wear. Conservative clothing is mandatory, which means that you must cover your knees, shoulders, and chest. Women should stay away from form fitting clothing, especially when in public. However, there are exceptions to these rules such as in American hotels where I saw many women wear form fitting jeans. Also, if you visit a “compound,” rules usually don’t apply at all.
As a female tourist, an abaya (head to toe coverage) is not required nor is a hijab (head scarf). However, when you’re visiting the airport or in town, I would recommend covering your head and body as much as possible for your own comfort as well as to show respect to the locals when you visit Saudi Arabia.