This is a good site for ideas:
http://www.travelportland.com/
Since you will be coming in on a holiday weekend, I like your plan to basically stay put. There are many very good things to see and do very close to Portland, but freeways tend to suck on holidays and speed enforcement tends to be very tight. There are lots of things to see and do that are convenient to the light rail and trolley lines; see
http://trimet.org/ for details.
The Washington Park Zoo is very nice, although likely to be quite busy. The Rose Gardens are spectacular that time of year as are the Rhododendron Gardens over by Reed College (also worth walking around). I always send people to the Pittock Mansion if for no other reason that you can see all the major peaks from Ranier in the north to Mt Jefferson in the south. Quite spectacular (
http://en.wikipedia.org/wiki/Pittock_Mansion).
Of course, Powells City of Books is one of the largest bookstores in the world and a must. A stroll along the waterfront and maybe a trip on a sternwheeler is always a blast. Don't forget that OMSI (omsi.org) has a decommissioned nuclear sub (the Blueback, I think).
But Portland is known for beers and food. Don't go to any of the microbrew chains like McMennemins or such, but get thee to a real bar like Kelly's Olympian (
http://www.kellysolympian.com/). There are some really cool eateries for any meal you may want at any time of day (just watch some episodes of Portlandia). For real good ideas, take a look at the Willamette Week website (
http://www.wweek.com/portland/index.php), especially the food and entertainment section. They cover Portland better than anyone. When the weather is good, I would recommend heading to any of the major food cart locations and just go for it. Note: food carts can't yet serve alcohol, but it's up for discussion with the city council, so maybe by May.
The Mark at the Shilo Inn on Canyon is the place for smoking cigars. The cigar bar at El Goucho is now closed and the Horse Brass quit allowing smoking 2 years ago. This is a decent web site,
http://cigarplaces.com/city/portland-cigars.aspx, but I'd always call a cigar bar listed there just to be sure before wasting time heading over there.
Finally, watch this before you go so there are no surprises
http://www.youtube.com/watch?v=AVmq9dq6Nsg