Portland has a good public-transportation system, which consists of local buses, streetcars and the MAX light rail. All are run by TriMet, which has an information center at Pioneer Courthouse Sq.
Tickets for the transportation systems are completely transferable within 2 1/2 hours of the time of purchase. Buy tickets for local buses from fare machines as you enter; for streetcars, you can buy tickets either at streetcar stations or on the streetcar itself. Tickets for the MAX must be bought from ticket machines at MAX stations (before you board); there is no conductor or ticket-seller on board (but there are enforcers).
If you're a night owl, be aware that there are fewer services at night, and most stop running at 1am; check the website for details on a specific line.
For an eco-friendly transport option, contact one of the several pedicab operators in town, including PDX Pedicab.
For custom bus or van charters and tours, try EcoShuttle. Its vehicles run on 100% biodiesel.