First Anniversary Dinner in Downtown Portland
"Great food" - that narrows it down ;)
Clyde Common, Gruner and Ping woud be my choices for "downtown" (well, the first two are west end and the latter is old town, but I'm guessing you mean broadly downtown, not literally downtown). If you're willing to stretch to the Pearl (and really it is an extra 5 minutes' walk), I would add Andina, Metrovino, Park Kitchen and Irving Street Kitchen.
Of all those - I think Andina is a great "special event" kind of place, though it can be loud and, depending on where you're sitting, not always intimate. Clyde IS as long as you get a table on the mezzanine (not the communal tables - unless you don't mind that) and not one right near a speaker. And as long as it's not very busy, Park Kitchen has a great small, tucked away feeing and is never too loud.
I would check what is actually open on a Sunday - now I think about it, Gruner and Ping probably aren't.
1014 SW Stark St, Portland, OR 97205
422 NW 8th Ave, Portland, OR 97209
1314 NW Glisan, Portland, OR 97209