If you want access to the Atlantis without actually having to stay at (or pay for) the Atlantis, the Comfort Suites is your choice. 
It’s just a five-minute walk from its behemoth neighbor hotel complex, and your key card gets you access to the Atlantis’s beaches, attractions, and water park as well as signing privileges at the restaurants—this, by the way is a $139 value in of itself (the price of an Atlantis day pass). 
The rooms are fitted with slightly old-fashioned modular furnishings, and many have a vaguely Japanese-looking half wall of frosted glass panes to divide the room into sleeping and sitting areas.