As long as it is "room-only" reservation and not a package, you can check out early at any time. Now if the days you want to drop are the first two days, then you have to do that in advance.
I've done it..
It was a room-only reservation. They didn't let me drop the days at check-in, but I just went to the desk when we were ready to leave and they refunded the "dropped" days - no problem.
I've done it..
It was a room-only reservation. They didn't let me drop the days at check-in, but I just went to the desk when we were ready to leave and they refunded the "dropped" days - no problem.