That’s a big area but there are a lot of options. It’s good that you say you’re not big city people, because the Willamette Valley is short on big cities but big on quirky smallish cities and towns.
Here are a few broad ideas about things you might like to see and do as you make your way from Portland to Eugene:
Wine/Beer Tasting: The Willamette Valley is particularly known for having wonderful wine, craft beer, and great food. In fact, it was named the #1 wine region in the world this year! If you’d like to do some wine tasting on your way down, I’d recommend taking Highway 99W south from Portland to Eugene. It will take you a little longer than staying on Interstate 5, but you’ll be driving right through the heart of the wine region. A middle of the week trip will be a good time to do wine tasting too, as it won’t be crowded–just make sure you check ahead with smaller wineries, as some may not be open on a weekday, although most are happy to accommodate with an appointment if they know you’ll be stopping by. You can find some wine touring ideas here.
Just like the Willamette Valley is big on wines, it’s also known for great craft beer; check out this site for some ideas of Willamette Valley breweries. If you take 99W south, about midway between Portland and Eugene you’ll hit Corvallis, which is a great spot in the microbrew scene.
Evergreen Air & Space Museum: Another spot to visit that’s just off 99W! This is a very cool museum all about the history of air travel and space flight. It’s home to lots of historic aircraft, including the Spruce Goose, built by Howard Huges in the 1940s, made entirely of wood, and still the largest airplane ever constructed.
Covered Bridges tour: Another fun thing to do as you’re making your way south would be to get off the beaten track with a covered bridges tour. One of the nicest loops to drive or bike, which will take you past half a dozen picturesque bridges, is just outside Albany.
Willamette Valley Scenic Bikeway: The nation’s very first designated bike touring route runs through the valley. You could rent bikes and ride a portion of the bikeway if the weather’s nice. This has info about bike rentals and touring assistance: http://rideoregonride.com/resources/
Silver Creek Falls: One of the most famous hikes in Oregon, but it shouldn’t be too crowded if you go on a weekday in March. Plus it’s famous for a reason! You get to walk over, under, and behind 10 waterfalls on the Trail of Ten Falls.
McDowell Creek Falls: Another great waterfall hike that’s a little more off the beaten path. And if you go on a weekday in March, you probably will have the entire trail to yourselves. This is a good one to combine with if you do the Covered Brides tour, as it’s in the same area of the valley.
McDonald-Dunn Forest: This is the forest you’ll find me in most weekends. It’s an entire forest managed by Oregon State University for their forestry research, but it’s open to the public for everyone to enjoy. Truly a gem, with trails for hiking and mountain biking that criss-cross throughout it. If you’re driving south on 99W, you’ll literally drive right by it! Even if you just stop and do a quick loop to stretch your legs, it’s worth it.
B&B recommendations: Youngberg Hill or the Black Walnut are both gorgeous inns on the site of working vineyards. Another fun place that isn’t exactly a B&B is The Vintages, a place where can rent a vintage Airstream trailer for the night. The Hanson Country Inn is another good B&B that would be a nice place to stay if you were interested in checking out the Corvallis beer scene.