I was just up on Orcas last month and had some great food. Cafe Olga (http://www.yelp.com/biz/cafe-olga-olga) is always a favorite of mine. It's tucked inside Orcas Artworks. Had a delicious Dungeness crab quesadilla and blackberry pie for dessert. There's often a wait, but it's fun to wander around the store and check out all the work by local artists.
Just up the road from there is Buck Bay Shellfish Farm where you can get fresh oysters, scallops and salmon. We took our oysters back to our rental house where we were fortunate enough to have a smoker, but I saw some folks shucking oysters on a picnic table outside too after buying. Beware that their hours are limited though.
I also discovered Mijitas (http://www.yelp.com/biz/mijitas-eastsound) this trip and had a delicious chopped salad with grilled shrimp that had an awesome smoke flavor, and others in our group had the beef short ribs that were amazing. Great happy hour margaritas and chips/guac/salsa. I have to admit that I didn't have high hopes for getting good Mexican food up there, but was blown away by how great everything was. Super friendly staff too. Would have definitely gone back for a second meal there if we'd had the time.
Haven't been to Allium, but second PAO's recommendation on what I've heard from others that it'd be worth checking out.
If you're going to be there over the weekend, be sure to check out the Farmer's Market in Eastsound on Saturdays. There were several food vendors that were worth checking out including salmon tacos and BBQ sandwiches.
Although I haven't been there in years, the restaurant inside the Inn at Ship Bay (http://www.innatshipbay.com/dining_ev...) was always a great place for dinner.
Also, add to your to-do list to drive/bike/walk up to Mt. Constitution for some fantastic views. It would be fun to pack a picnic lunch to take up there too. The grocery store in Eastsound is well stocked.
The owner of the house where we stayed also recommended Sazio, Madrona Bar & Grill, Rose's Bakery, and the Lower Tavern, although I personally haven't been to any of those.
Have a great trip!