Starwood Preferred Guest's Q3 promotion is open for registration if you have not done it yet.
The offer runs between 1st October – 20th December 2015 with registration required by 6th December 2015.
You will receive double the Star Points on stays of 2 nights or more, plus an extra 500 for stays of 3 nights or more. Logically to maximise on this promotion your stays need to be exactly 3 nights long. Terms and conditions to the promotion found here.
To calculate the base points, you gain 2 Star Points per USD. For bookings in other currencies, you will need to convert to USD. The bonus star points you get will be 2x the base points. If you are a gold or platinum member you get an additional 1 Star Point per USD.
The promotion this quarter is fairly boring and doesn't scream mattress run potential, but as with most things if your employer or someone else is picking up the tab it can make a noticeable difference to your SPG balance.
The opt-out list of hotels is not large, though not small either.
If you have little idea on how SPG works, I wrote a beginners' guide a while back. The information on there still valid.