The International Rose Test Garden (in NW Portland) is one of my favorite places to visit on a Sunny (or not) day. The roses aren't quite blooming yet, but there is still plenty of gorgeous flora to appreciate and it can be a wonderfully relaxing & peaceful walk (go in the early morning and you'll just about have the garden to yourself). I usually walk up Burnside towards NW 23rd and then a few blocks past 23rd I take a short path off to the left (with stairs) that leads up to the garden.