Welcome back, Jennifer! My daughter and I loved our time swimming with the dolphins and enjoying the beautiful beaches of
Blue Lagoon. I highly recommend this adventure for your group!
I do not believe you would be able to go to both Blue Lagoon and to Atlantis in one day, as that would cut pretty close on time, not allowing you to really relax and enjoy. As you check in for your cruise online, you'll be able to view times for the various adventures and choose them. The system will only allow you to book two when they don't overlap. The nice thing about both Blue Lagoon and
Atlantis is that they offer returns throughout the day, so you can simply head back to the ship when you're ready. I am not sure of the exact times of departure from the ship for the adventures, as they vary based on sailing, but I do not believe you'd be able to be back to the ship in time to then head out on the second departure for the next adventure. Perhaps your group may want to split up for the day to each enjoy the adventure that is the favorite for them.
If you have to choose between the two, having been to both Blue Lagoon and Atlantis, I'd recommend Blue Lagoon over Atlantis for the dolphin swim and beach day. But if it's just the beach that is drawing you to Blue Lagoon, you won't find one better than the day you'll spend at Castaway Cay. So, another thing to keep in mind is that you could enjoy the dolphin swim and gambling at Atlantis, and save your beach day for Disney's Castaway Cay which is the best of the bunch! Sorry I can't give you a definitive "DO THIS" answer, but I hope the information I gave you points you in the direction to choose the best day for your group!
Good luck choosing, Jennifer!
Marcy