An afternoon doesn’t give you much time, so I’ll limit this list. Were you to be staying longer, this list could grow quite a bit.
Deschutes in downtown Portland (actually the Pearl, but that’s not important) is definitely one of the best places to consider. You may well know Deschutes, but it’s really worth visiting the local location here. They have a small brewery on site, and they have something like 22 taps, most of which serve beers you’ll never see outside the pub. Deschutes has always had, from top to bottom, one of the best taplists in the country. Great variety of styles, all excellent quality. If you’re looking to eat, they do nice food, too.
The other big brewery to consider is Breakside. It is a little bit remote in the city (out to the north of downtown), but no brewery is making better, more interesting beer in Portland. Like Deschutes, they do a lot of different styles, so you’ll definitely find something there of interest no matter what your tastes. But they do specialize in IPAs, and they’re at the forefront of the modern palate (lots of juicy, fruity flavors). They’re one of the most influential breweries in Oregon right now. They also have a great menu.
There are a bunch of other excellent breweries, and you could spend a week trying to go to them all. Other good choices are: Ex Novo (nice selection of different styles), Upright and The Commons (farmhouse beers), Cascade (sours), Hair of the Dog (big, boozy beers), Ecliptic (standard ales, by the original brewer at Deschutes), Hopworks (standard ales), Lucky Lab on Hawthorne (somewhat average standard ales, fantastic location), Occidental and Zoiglhaus (German beer styles), Widmer (huge selection of beers you can’t get elsewhere).