I live in Portland and Powell's Technical Books is a good suggestion. It's a branch of the main Powell's store which is located a couple a blocks up the street.
I always recommend that people visit Pittock Mansion early in their trip. It overlooks the whole city and the surroundings of the city (Columbia Gorge, Cascades, Mt. Hood). It's a free thing to do that has tremendous payoff. It allows you to get a good handle on the layout of the city in one fell swoop. Makes the rest of the trip easier. Note: I'm not talking about going IN the mansion.
Free Geek is a technology-only thrift-store. There's room for improvement but it's pretty great. I buy stuff there often. http://www.freegeek.org/
There are A LOT of tech-related meetups that go on here. Check upcoming.org for your flavor of discussions.
Mostly, I recommend just chilling out a coffee shop like Stumptown.