We just returned from the 5/22 Wonder 3 night. (My trip report will be finished soon.)
We did the same thing...took a cab from port to Atlantis. Since we had no excursion tix, our Atlantis exploration was limited to the casino, the shops, and a lobby view of the aquarium. We did find a deli for lunch and an ice cream shop near the marina.
All access to the grounds, beaches and pool areas were guarded by staff who required us to show a guest key card before leaving the public areas. Of course we couldn't comply, so our Atlantis adventure lasted barely 90 minutes and was limited to the indoors. I was kinda bummed that we couldn't even walk around the grounds.
If any members of your party want to see the pools and beaches, water sports, etc, I'd highly recommend they book the excursion or rent a day room. They won't even be able to get close to water activities without them. Keep in mind, however, that Disney excursions only provide access to the grounds and beach areas. They DON'T provide the wrist bands necessary for swimming in the pools.
I understand day rooms at Atlantis are rare, but the Comfort Suites across the street might be a little easier to book. Comfort Suites guests get all the same access privleges as Atlantis guests. Last time I looked, a room at CS was $175...about the same price as a
DCL excursion for 4.
Hope that helps.
Kelley