Hi Shelby,
Sounds like you're going to take a wonderful vacation!
I rather like your plan. Since this is an adults-only trip, you should be able to really maximize the two days that you do have in the theme parks. By going to two parks per day, you'll be able to get to all four theme parks. I do suggest that you check the schedule for
Extra Magic Hours while you are there. Using Extra Magic Hours, as guests staying on Walt Disney World property, on select days you can enter a certain theme park early or stay late. If you go to your first park for the morning Extra Magic Hours, or stay at the second park for evening Extra Magic Hours, you will extend your time in the parks.
I do like you idea of having a day at Downtown Disney. You can do some shopping and eat at some great restaurants while you are there. If this is on or after your 21st birthday, you can celebrate at some of the bars there as well!
Raglan Road would be a fun choice. Also on your Downtown Disney day, consider spending some time at your resort as well. Disney's Port Orleans Resort - Riverside is lovely. A stroll around the grounds and a dip in the pool would be a relaxing way to spend part of that day.
Be sure to get yourself a birthday button at Guest Relations at one of the theme parks. These buttons are available free of charge. Wear it around the parks, and you never know what special surprise you might get, like a cupcake added to a meal. Don't expect these perks, but they might occur. Even if they don't, it's really fun to hear cast members and guests alike wishing you a happy birthday.
Have a wonderful time celebrating, Shelby! Please do not hesitate to ask if you have any further questions.