Avis has a Skagwag newspaper that gives you mile by mile sights to see. In my opinion, the road beyond WhitePass was the best, i.e. up in British Columbia. I would recommend going all the way to Emerald Lake (roughly 75 miles).
I'd really recommend buying the Milepost. It's specifically written for Alaskan/Canadian roads and describes anything of interest using the roadway mileposts as designators.
It beats anything else on the market (especially AAA) and makes a super wishbook for future Alaskan adventures!!
PS - They are updated annually and available through amazon.com and barnesandnoble.com.
I just returned and made the drive from Skagway all the way to Whitehorse. The scenery is beautiful along the way. Emerald Lake is a must see as others have told you. It was about 130 miles from Skagway to Whitehorse. We were back in Skagway by 3:30 and left after breakfast. Make sure you have a full tank as the only gas station you will pass is in Carcross. I wasn't impressed with Whitehorse although it was a fun ride. Most of the scenery is in the first 50 miles of the drive. We did spot eagles on the way. If you drive out and stop on the way back you will avoid the buses who make thier stops in the reverse order. There is only one road out of Skagway accross the bridge. Stay on it and you will find it impossible to get lost. You might want a Yukon Territory map, as this portion was not on the maps I received in Skagway. Feel free to email me if you want more info. We drove a vehicle from PB cruisers, 52.00. It was old but served the purpose. If you are using PB cruisers, they moved their office right down the road to Franks carryout.