If I traveled to California enroute to inspect my rental property in Hawaii but decided to return home because Covid hit, can I deduct the CA. airfare as rental expense?

Probably not- per the IRS

"You can deduct all of your travel expenses if your trip was entirely business related. If your trip was primarily for business and, while at your business destination, you extended your stay for a vacation, made a personal side trip, or had other personal activities, you can deduct only your business-related travel expenses. These expenses include the travel costs of getting to and from your business destination and any business-related expenses at your business destination."

Because you did not make it to Hawaii, the stop in California, although not the original intent, became the primary purpose.  If you did not have family in California and it was just a stop on the way, it may be different.