I believe the signed route is set to avoid River Road North through Keizer. It does a good job of sticking to bike friendly roads. There's a bike lane on most of Commercial St in Salem so no biggie there. If you are on Commercial Street, just past the library turn right at the gas station to avoid a short narrow stretch. I believe that is signed also. River Road South is one way to get to Buena Vista. It would give you an option of cutting over to Independence or wandering around to Ankeny and the ferry. Once you get past the bridge to Independence, it is a little narrow in places with some rollers but very low traffic. There are other routes to Buena Vista you might use that may involve climbing depending on which road you take.

There are alot of merry go rounds in Salem. Which park is it? They might have meant the carousel at Waterfront park.

The stretch from the ferry to Corvallis should be nice. Sometimes it gets really windy from around Harrisburg to Eugene depending on the route.