SPG has a program in place where guests can opt to decline housekeeping services. In return SPG will give you either 5$ credit or 500 SPG points per night.
Details can be found here or here (There are different links for Westin and Sheraton)
Four Points properties also appear to participate in the program, however they only offer you a bonus of 250 points or 5$ dining credit. Details can be found by searching for the name of the specific Four Points property and "make a green choice"
The less water, energy and other resources we use, the more there is to go around. Reward yourself with a $5 voucher at participating food and beverage outlets or 500 Starwood Preferred Guest Starpoints® awarded at check out for each night you decline housekeeping services (except day of departure).
Conservation matters. Make A Green Choice