My thoughts are: it's a special day and you can afford to splurge on real cake.
But if you feel the need to do something low-cal, how about angel food cake topped with strawberries and cream/whipped cream?
There are several blogs/websites out there that have recipes for low-cal versions of cake. Hungry Girl and Skinny Taste are two that I know of but I can't vouch for any of their recipes since I haven't made any of them.
Edit: Happy early birthday!