Free parking may be a tall order in downtown area. Cheaper parking than the hotel rates is probably easier to find. There are several lots east of the hotel along Queen's Quay that serve daytime visitors taking the ferry out to the Toronto Islands (ferry dock is right next to Westin) - and they typically charge flat rates for the day that get lower the further away they are from the dock ($10-$25). But for overnight, I would recommend seeking a Green P lot (city owned parking authority) for lots that are more secure, rates that are less gouging and more easily paid for by credit card (most of the private lots are cash only). This website shows you where the Green P lots are, and their rates:
https://parking.greenp.com/find-parking/
They appear to have a few lots a few blocks east of the Westin on the south side of the Gardiner.