The best way to get around the city is definitely via bike. Portland is a super bicycle friendly town, and if the weather's nice it's a great way to see the city while serving as (cheap) transportation. Most hotels (including the Ace Hotel!) offer bicycle rental. Otherwise, try
Clever Cycles,
Waterfront Bicycles, or
Veloce Bicycles (to name a few).
Clever Cycles, 900 SE Hawthorne Blvd, 503/334-1560
Waterfront Bicycles, 10 SW Ash St #100, 503/227-1719
Veloce Bicycles, 3202 SE Hawthorne Blvd, 503/234-8400
[Photo via @burps]