Just to note, the free rail zone no longer exists in Portland. All fares are $2.50. The Max is a great way to get around and you could easily pick a hotel along a line and use it even late at night. Just check the schedule for your stop as some stops are very infrequent super late at night or before dawn.

And I second Evergreen. It's an awesome museum.

If you like railroad, check out the ORHF museum opening up in Portland this month. A few very cool old engine and a lot of other train bits. There is also a Mt. Hood scenic railway you can ride. I have seen the train but I haven't been on it so I can't say how fun it is to ride.