You're coming to the right place!  There are lots of outdoor activities to enjoy here, and I'll let others chime in (especially cyclists and wind/kite surfers).  My own passion is kayaking, and you can rent single/double kayaks and canoes many places to see the sights from water-level.  

A favorite is Willamette Falls (rent from NW Outfitters/NRG Kayaking in Oregon City), or you can go around  Ross Island with great city and natural views, including the houseboats of Oaks Bottom (rent from Portland Kayak Company on Macadam). Nearer the Columbia River, you can float the Columbia Slough and drop into the Island Cafe on Hayden Island (rent from Alder Creek Kayak & Canoe).  Farther afield, you can paddle the Scappoose Bay ( Scappoose Bay Kayaking)  There are lots of water adventures in this region - these are just a few of my top picks….

Ok, Lissa has kayaking totally covered and she's right! Kayaking is amazing in Portland with so many rivers running near it or through it.

I grew up doing intertubing and white river rafting along the Clackamas River - there are some great areas depending on your skill/adventure level with various grades of rapids and nice easy calms. I usually started near Molalla or Clackamas and went from there.

Try http://www.blueskyrafting.com/clackamas.htm or http://www.riverdrifters.net/clackamasriver.htm.

For hiking/backpacking, go to the Columbia River Gorge National Scenic Area. There are countless miles of trails of various difficulty through this area along the river full of lush green and waterfalls. You'll LOVE it. You can spend days here or just one. Head towards Multnomah Falls and stop at any of the waterfalls along the historic highway and you'll  find trailheads. If you want to plan ahead, check out the site here: http://www.trailsnw.com/gorge.

Just up the river happens to be the Wind Surfing Capitol of the World!  True. Hood River is known for the wind and you'll see the river there full of beautiful sails. Look into it. It's a great place to wind surf.

Of course, where there are rivers, there's water skiing... and you can't forget the snow either... Head up to Mount Hood National Forest and go snow skiing. Yes, you can ski at Timberline Lodge during the summer on a glacier. How cool is that?!?

Check out Multnomah Falls. Its within an hour drive and stunning.  Also, their are locks on the Columbia River that you can see Salmon which is impressive, but usually only in the Spring.

Another place to see for greenery is Sauvie Island. 

I'd definitely recommend hiking in  Forest Park and cycling on Sauvie Island (you might hit one of the fruit seasons just right and be able to pick your own). Portland is very much a city but Forest Park in particular will give you a taste of the Cascades. Just be sure to get a map, it's very large! There are beautiful lakes and trails around Mount Hood National Forest, you might consider taking a loop through Hood River and around the mountain, stopping to explore along the way - don't miss Multnomah Falls!

In town, check out  Forest Park or Hoyt Arboretum

These are great for hikes.  On the east side, try  Mt. Tabor Park.  It's got a great view of downtown from the top, and you can visit the crater as well. Yes, it's a dormant volcano!

East of the City is the not-to-be-missed  Columbia River Gorge National Scenic Area.  Tons of hiking here (be prepared!) and interesting towns, like  Hood River when you want to quaff your thirst or grab a bite.  Speaking of Hood River, you two might want to take up windsurfing on the Columbia River while you're there!

The river floats are the best! You grab an inner tube, some snacks, a beer or two, and float down the Willamette River with a huge group.http://www.thebigfloat.com/

Hit up Multnomah Falls! Nothing like it back in Michigan! Multnomah Falls