In the heart of Portland's Pearl District sits one of my favorite "special occasion" restaurants, Andina. Great tasting food, interesting ingredients and wonderful presentation. Getting lots of small plates to share is a great way to try dishes you've never had before.
The cocktails here are delish...as other reviewers have said, I highly recommend the Sacsayhuaman, a passion fruit puree and habanero-pepper-infused vodka delight. Also notable are the pina colada (one of my dinner companions said it was THE BEST), and the sangria.
I also recommend ordering some smooth and delicious peruvian coffee with your desert. You get it served to you in your own little cast iron coffee pot.
TIP: The bar area is pretty loud, but has lively atmosphere. There was a bongo drum player entertaining us the night I was there. For easier dinner conversation, make reservations in the dining room, although the acoustics there still make for a fairly noisy environment.