I recommend a full day of farting around Astoria, as there are several cool things there (The Column! Maritime Museum!) and has a decent brew pub (Fort George) and another ok one too. As for riding, if I had a day and was travelling to/from town by car, I wouldn't bother. Astoria is a small enough city that you can park and walk everywhere easily. The riding in the city, if not sticking around the waterfront, is up and down steep San Franciscoesque hills. And pretty much any way out of town involves a busy highway. They're not too bad in a bike touring sense, but for just a joyride I wouldn't be interested. Of course, I don't live in Astoria, so someone who lives there (paging fietsbob) may have a different perspective.

It's about 80 miles one-way from Portland to The Dalles, and you'd have to use that section of 84 to get to The Dalles. You could break it into two days and overnight in Cascade Locks, which isn't as "happening" as Hood River, but lodging should be cheaper and there's a good brewpub (Thunder Island.) Across the river in Stevenson is Walking Man Brewing.

One thing to note about the Columbia Gorge is the wind. During the dry season it typically blows west to east, and gets stronger the further east you go. If you intend to go out and turn around, you'll eventually have to deal with a pretty stiff headwind. Especially if you venture as far as The Dalles.

Don't know much about the bike scene, but it is a big outdoor recreation spot. Deschutes Brewing (Black Butte Porter, Mirror Pond Pale Ale, etc) is based there, and I'm sure there's a few other brewpubs as well.