I just logged into the Marriott Rewards website and saw that Marriott has a new Megabonus promotion which you can register for. This Megabonus lasts from March 1st until May 31st.
The way it works is that you get a certain number of bonus points for a certain number of stays. In my case, I got offered 5,000 bonus points for every third stay. Every time you check-out, one stay is counted- regardless of the actual number of days stayed at the hotel. This can add up pretty fast, especially if you are going to be staying at several different Marriott hotels.

It costs you nothing and takes a couple of seconds to sign up for this offer, I strongly suggest signing up.