OMSI is definitely worth seeing. Very fun place. Powell's is awesome, and there is a lot of shopping that has sprung up in the Pearl District around Powell's.
If you are of age and enjoy beer, then there are lots of microbreweries to hit up in the Portland area. There are also a lot of good coffeeshops too
Portland is all about the coffee and the beer.
For exploring downtown, I would seriously recommend getting a day transit pass that you can use on the MAX, the streetcar, and the bus. The streetcar goes from downtown up through the Pearl and over to 23rd Street, a very hip and fun shopping area. The MAX goes through downtown and will take you to the zoo. A bus can take you across the river to OMSI, or you can drive. If you are going to park downtown, I recommend the SmartPark garages:
http://www.portlandonline.com/smartpark/ because they're a LOT cheaper.
It's too bad you're visiting in wintertime. Portland is much better in the spring and summer, when the roses come into bloom. Washington Park and the International Rose Test Garden are beautiful when the weather is good. You'll also be missing out on the opportunity to see the Portland Saturday Market, which is definitely interesting (especially for the people-watching).
If you like old, historic homes I highly recommend touring Pittock Mansion, or at least driving up there for the view of Portland; it's amazing. If you like gardens the Japanese Garden is worth seeing, and I've heard that the Chinese Garden is also good (and it's downtown in Chinatown). That's all I can think of at the moment.
As for bars, I fully recommend Ringlers and Kells. If you want 24-hour food, there's this weird joint called the Roxy on Stark St. They have the best cheese fries ever. Oh, and if you like drag shows, it's worth seeing Darcelle, just for fun. Horribly cheesy...but still fun.