i use their online shopping portal which gives me extra marriott points plus the points for using the credit card for stuff i would buy online anyways. I do have hertz but other car rental places are much cheaper although marriott does sometimes have promos for 2k or 5k bonus points with hertz. If you have a business, i would definitely try their portal since they have 2pnts/$ or more at staples, homedepot, lowes, best buy, fedexoffice, ect.
I will echo the reponse by jm1991 and also add that I frequently make purchases through the shopping portal ( www.marriottrewardsmall.com ) that don't need to be shipped. Several merchants (such as Office Depot, Lowe's) offer in-store pickup of your order. I usually get more points per purchase by doing that, plus my order is ready for me when I get to the store.