You can walk around Atlantis without a day room, but you have to pay to go through the dig, or to use the pools, their private beach, or water slides.
Comfort Suites also does day rooms at a little cheaper rate, that will give you access to all the stuff.
I think my friend paid over 200 for the day room, and we really were so pooped, as they put the room out in the back forty, and we kept getting turned around, that we really didn't enjoy it that much...You need to specifiy that you are a cruise passenger looking for a day room at either place if you decide to go that route.