What do you mean side drive? Aren't you spending about a month in AK?
I wouldn't drop big $ on the ferry, although it was super cool. We took it from PR to Haines and most of the entire boat ride was just spectacular scenery.
If you want to take a boat ride, I'd go fishing for a day. At the end of the day you'll have $100s worth of fresh fish to eat.
Man, so many things to do. We were up there for a year (living and working, not just vacation) and didn't see the half of it.
Off the top of my head:
-Loop form Whitehorse thru Dawson and Chicken.
-Haul Road to Coldfoot or all the way to Deadhorse
-Denali Hwy
-Kennicot mine/MacCarthy
- Valdez
-Kenai Peninsula
Too much to type. If you need suggestions or a couple fishing holes, PM me.