If that is out, then I would suggest riding Sauvies Island. It's a mecca for all types of cyclists. Very peaceful, flat, and cyclist friendly. Park at the "Cracklebarrel" store over the bridge. Our club, the Vancouver Bike Club, will usually leave from Vancouver and ride out, do the main loop and back. It's about 50 miles, so you'd be a little short, but you could do more than the loop by heading off on some of the roads that dead end after several miles. Grab something to eat at Krugers.
Here is the map and cue sheet we use. http://www.vancouverbicycleclub.com/roadscholars/Maps-Cues/RSSauvieIslandC.pdf
Also we do a "French Toast" ride, that leaves in Vancouver and loops you around Portland following I-205, Springwater Trail, and city streets through town, hooking up to I-5 back into Van. You'll see the city and would be on bike paths most of the way, but it's a little more hectic and busy. Still a good way to see some of the city.
Map and cue here...http://www.vancouverbicycleclub.com/roadscholars/Maps-Cues/RSFrenchToasts2.pdf
And to do my part as club member, I'll invite you to check our calendar (even though it's not filled out for April) to see if we have any club rides that might be interesting if riding with a club floats your boat. http://www.vancouverbicycleclub.com/index.html
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