Ooh, I love birthdays at Disney! Usually we (whoever I'm going with) will let the birthday person choose the main activity for that day (so like park, water park, something else?) and build on that.
For example, my mom loves Epcot (especially food and shopping in the European countries) and Ohana. One year we started with breakfast at Ohana, then headed to Epcot, did whatever she wanted, had dinner at Via Napoli (started earlyish so we could eat at a leisurely pace but not miss the fireworks). And then had birthday cannolis while we watched Illuminations.
I'd start by thinking of what your favorite things to do at WDW are. If you can't narrow it down, then try something different or that you've never done before because you haven't had time or didn't want to splurge or what have you.
