Are there any special restaurants or WDW opportunities (e.g., the Wishes cruise or dessert party, the Characters in Flight balloon, the Circ de Soleil show, etc.) that you've been wanting to try?
Could you get a breakfast reservation at your favorite park before opening, so you could enjoy some quiet time in your favorite park before opening?
Could you do something silly or fun, specific to your interests, like using a park hopper to see all of your favorite attraction(s) in each park on your birthday?
Depending on your kids' temperaments/your comfort level, could your kids check out one of the Kids Clubs while you and your DH have a grown-up evening?
If I were you, I'd be really tempted to order a birthday cake from WDW because they always look so good in pictures!