You are going to get tons and tons of ideas like BBB and such, but I would think simple if it were my child.
Let your child pick the park and the rides for the morning that she wants to. If she wants to meet characters, let her. Then pick a special place to have lunch and ask for a birthday cupcake (cake is way too much both to eat/cost). Then I would take her back to the resort, rest, do the pool, and get an easy dinner. If she stays up late, I would take her back for the fireworks at the park she chose. Buy her a firework light-up toy, watch the fireworks, call it a day.
Sometimes simple is better and I am sure she will remember it more if she is not overtired and stressed.
Have fun.