Hi khurley23!
I'm also celebrating my 40th bday at Disney this year! I'm planning two separate meals to celebrate my birthday (though of course the whole trip celebrates it!) I'm planning a date night with my husband at California Grill, I've (hopefully) timed it so we can watch the Wishes fireworks show right at the end of our meal. I'm also planning to celebrate with my husband & kids at Be Our Guest for dinner one night. I'm thinking of having a cake delivered to our room one day, rather than having it a dinner, as I don't want to deal with carrying the cake leftovers around the parks with us, as I'm sure my family will want the leftovers!
There are so many great options for a treat. La Cellier is a good option, but I think you should also consider one of the signature restaurants outside the parks. The signatures restaurants cost you 2 meals per person on the dining plan, but they are excellent meals. We have enjoyed Narcoossee's at the Grand Floridian in the past, as well as Yachtsman's Steakhouse at the Beach Club--of the 2, I preferred Narcoossee's.
Still on my list to try is Artist Point at the Wilderness Lodge--I've heard wonderful things about it.