It's too bad your visit wasn't a week earlier! This week's weather has been nicer than a lot of the summer was. I wouldn't expect any downpours next week, but this time of year "rain" tends to mean light on-and-off showers for half the day, meaning rain gear will be key. The gorge is a lovely place to ride. Multnomah Falls is about 25-30 miles from Portland, and it's a beautiful day ride out and back -- even in the rain. (Make sure you have good lights for being seen on the winding road.) Cyclocross races (in costume on Halloween) will be in Astoria (2 hours west of Portland) the 30th and 31st.

If you go hiking in the Gorge, I love the Angel's Rest hike. It's a few miles west of Multnomah Falls, with a great view once you get to the top.

If you'd like to borrow a clunkyish but not-at-all-attractive-to-thieves bike for riding in the city, I'd be happy to lend you mine for the duration of your visit (whether or not you choose to bring your own bike). Send me a PM if you're interested!