While in Oregon City there are a few places that I would recommend to smoke some marijuana. One of the best and least visited is the Willamette Falls area. The Falls themselves are quite majestic even though Industrialization has pretty much ruined the view. But it is very peaceful there, not much traffic and you can get some great pictures of the falls. Up the road there are some restaurants to eat at afterwards as well. But this is probably your best bet for a relaxing afternoon of smoking some weed and taking in the scenery.
Willamette Falls has the best views for a truly unique display of natures creativity. There are plenty of rocks and grassy spots by the waterways you could chill out and smoke there. I would consider another park though if you want something very relaxing. Namely, Clackamette Park. The water is still there and it’s more of a place to bring a blanket and hang out for a while and do your thing. Check out the farmers market if you’re back in town and want to do some shopping. A lot of it is open air environment and you can smoke around there with respect to the people around you. There’s all kind’s of goods and stands with locally produced products.