Just to add to what GrnMtnMan said, the lobbies of the Beach and Coral towers are fairly ordinary. They are pre-existing tower hotels that got absorbed into the Atlantis complex when it was built. They're painted and decorated to fit into the complex, but the majority of the cool themed architecture and decoration that you've seen in pictures of Atlantis are in the areas that you need a pass to get into, including the Royal Towers lobby.
The cheapest way to just see the theming and architecture and walk around is to get a Discover Atlantis pass, which gets you into the Royal Towers, the Dig aquarium, and the predator lagoon. That's about $40 per person, last I checked. You don't get to see the stuff in the water park (well, a little bit from a distance), but the water park passes are exceptionally pricey, at like $130/person off-peak and higher during peak seasons.