Tax planning is usually best done with a tax professional that look at ALL of the details and probe you with a bunch of questions. But here is some general information.
Are the rentals completely separate units, with their own kitchens and entrances? If so, there really isn't a way to avoid paying tax on that portion of the sale (except, see note later about senior citizens). As was mentioned, the sale of your personal portion may be able to avoid up to $500,000 of the gain (assuming you are Married Filing Separately).
You didn't say why you want to downsize or your financial situation or if you would be willing to rent out the entire house while you move elsewhere. Those can affect some recommendations for what would be a good thing to do (and is why sitting down with a tax professional can be particularly helpful for tax planning).
You did mention you were senior citizens, and that you live in California (and said "we", so I am going to assume you are married). To keep things simple, according to current law, if one of you dies, you get a "step up" in value of the home. In other words, the 'cost' of your home resets to the Fair Market Value on the date of death (and all prior depreciation disappears). If the house was sold the day after one of you died, there would be zero tax due. So that could be a consideration about selling it.
