For your high end meal in Portland, ain't nuttin wrong with Higgins. Along those lines, there's also Wildwood and Paley's. The "new guard" high-end restaurants are also great, namely Sel Gris, Le Pigeon and Alberta St Oyster Bar & Grill.
On the reasonable end, go for Pok Pok, Kenny & Zukes, or Caffe Mingo on NW 21st. If you want to brave the lines, go for Apizza Scholls, but be forewarned that they may run out of dough as they only serve a certain amount per day, and the weather has started to warm up lately.
Neighborhoods you should check out while you're here: NW 21st/23rd (Wildwood, Paley's and Caffe Mingo are here), SE Division/Hawthorne (Pok Pok, Sel Gris and Apizza Scholls) and NE Killingsworth/Alberta (Alberta St Oyster Bar & Grill and others).
What's your budget, how many in your party, where are you staying, and will you have transportation?
This might help narrow your choices down, but it too only really scratches the surface, as ExtraMSG wanted to encourage people to do lots of exploring:
Don't miss Higgins for top-qualiity, authentic NW local food. Eat in the bar for the relaxed, bistro experience or in the dining room for white table cloth elegance. Interesting, carefully chosen wine list, fairly priced. Huge beer selection if that's your thing. Another winner is Equinox --wonderfully creative seasonal menu, always some delicious combo you would never have thought of. Also a nice outdoor space for good weather. They offer a list of inventive cocktails if you want drinks and small plates. For breakfast, try Bijou (have the French Feta Omelet) and if there's a long line (there will be), sit at the counter rather than wait for a table. Also breakfast at Cadillac Cafe, J & M Cafe or Zell's will make your tummy happy
Have fun in Portland -- it's a food-lover's heaven!