Make sure you mention your anniversary when you check-in at your resort and, if you do decide to celebrate at a restaurant, do the same there. You can also arrange a celebration cake by calling the restaurant itself ahead of time. When we did this, I think the cost was $16.
'Ohana had never appealed to me despite its popularity, but we ended up there for a dual birthday celebration in October 2006. Whilst it couldn't be described as romantic in the traditional sense, it does have a lovely vibe and it's certainly family oriented. You could try for a window seat to view Wishes, or (even better in my view), time dinner so that you can stroll down to the Poly beach afterwards to catch both the Electrical Water Pageant and Wishes.
Here is a photo of the two birthday girls and their celebration cake.
These two show our window location - it was right in the corner which I imagine is a pretty prized position, but as far as I remember, we were just assigned that table without asking.
It got dark as we dined which was nice.
Our American friend, Susan (SusanEllen on the DIS and the lady in the birthday photo) bought the leis and we all dressed up in our best Hawaiian/Polynesian garb.