The wedding is located in Lincoln City. There is plenty to do and explore.
While on the coast why not do a little fishing.
Who doesn't love cheese. On your return trip to Portland take a drive up 101 instead and make a stop at Tillamook Creamery - home of Tillamook cheese.
Drive north on Hwy 101 to Astoria. While there: climb the Astoria Column, visit all the sites where the Goonies was filmed, or stop at Fort Clatsop/Lewis & Clark Historical Museum.
Multnomah Falls is a must see and only a short 45 minute drive from Portland/Vancouver.
Take a scenic drive up through the Columbia River Gorge and stop to see waterfalls or go for a hike. Mary and Robert took this drive many, many times as they traveled back and forth to see each other between Vancouver and Moses Lake.
If you are a wine drinker, make sure to spend a day wine tasting before you leave. Robert took Mary wine tasting in the Willamette Valley for their second date (and her birthday).
Seattle is only 3 hours north of Portland.
Why not take a side trip to Mt. Saint Helens.
So many things to do and see. If you want to do a little shopping Oregon is a sales tax free state.