Hi Jodi,
What a wonderful way to celebrate a birthday! If you are hoping to involve dolphins for your Nassau experience, you've got a few options:
Blue Lagoon Dolphin Encounter - This would be my first choice. As much as I love to pull out all the stops while on vacation, I prefer to spread my "play" money out over a few excursions and souvenirs. At $125 per adult this is the least expensive dolphin adventure. I have heard wonderful things about this experience, and Blue Lagoon is simply gorgeous. This allows you approximately 20 minutes in the water with the dolphin.
Atlantis Dolphin Cay and Aquaventure - This is the ultimate port adventure. What is so appealing about this is that not only do you get 30 minutes or so in the water with a dolphin - you will also have complete access to the Aquaventure water features, pools, waterslides etc. It is worth noting that unlike the Blue Lagoon which takes place in an ocean lagoon, the Atlantis adventure takes place in their dolphin cay pool. This excursion is $295 for anyone ages 10 and up.
Blue Lagoon Dolphin Swim - This is probably what we could call a step above in comparing it to the in-water experiences of the two excursions above. What is unique about this adventure is that you will get to experience being propelled through the water by a dolphin foot push. The total time in the water is about 30 minutes, and the cost is $199 for anyone ages six and up.
I hope this helps, please keep in mind that prices are subject to change.