First, the ferry trip is great. The scenery is magnificent and it's a wonderfully unique experience.
A tip or two ... If you're traveling early in the season, May/June, then I suggest you ride up and ferry back. The northbound ferry's are pretty packed but the southbound trip I took in late June of this year had about a quarter of the Columbia's capacity. That meant no waiting for anything and only 3 tents on the Solarium deck instead of the crush that Antware shows. Besides taking the ferry at the end of the trip provides a welcome break from riding. Also, we stopped in Sitka on the southbound ferry. We didn't a few years ago on a ferry out of Bellingham. You'd have to check the schedule on that because Sitka is worth the stop....