Break out the different reasons you want to go to Alaska and what do you want to see and do. Most visitors go with some specific places they want to see and other treat Alaska as a place to make a U turn to head home.
With 6 weeks from Colorado, that gives you about 42 to 45 days overall. From Denver to Fairbanks is right at 3,100 miles so I would time budget that at about 6 to 7 driving days each way. Then I would add in a few stops such as a couple of days in Whitehorse, a day in Liard River Hot Springs Provincial Park, on the way up and then come back down the Cassiar Highway, Hwy 37, from the Junction 13 miles north of Watson Lake, YT, spend a couple of days on the Cassiar, taking the side road into Stewart-Hyder on Hwy 37A, three great provincial parks on the Cassiar Hwy, and some private ones as well. I would allow about 20 to 25 days for the to and from parts of the trip. Then the remaining time in Alaska. I also really enjoy driving the Top of the World Hwy from Dawson Town to Tetlin Junction, just east of Tok. Dawson Town is worth a day or two to many of us as it is a great history lesson of the Gold Rush days.
Doing the tourist circle will give a first or second timer a good taste of what Alaska has to offer a visitor. The Fairbanks region is my favorite as this is the part of Alaska I still consider to be home. PA12 has listed some of the stuff to do there, I would add a trip out to Chena Hot springs if time allows.
Then go south on the Parks Highway, stopping at Denali to view Mount McKinley and on to the urban sprawl of the Anchorage Bowl area, resupply if needed and head on south to spend a few days on the Kenai, Valdez is a real nice visit, one of our favorites, it is one of the only coastal towns accessible by highway, that is not over run by residents of Anchorage on the weekends. Still somewhat quaint, a couple of great boat tours, Stephens and the Lula Belle with Capt Fred, are both outstanding, I never get tired of the trips out to see the glaciers meeting the sea.
Then after you get off the Cassiar at Kitwanga Junction, go east to Prince George, resupply if needed, then head on east to Jasper and go south on the Ice Fields Parkway, probably not a more beautiful drive in North America, and on to the US-Canadian border, then head back to Colorado to start planning your return visit to spend more time in the places that gave you the greatest "wow" factors.
A lot seems to depend on where a person is from and where they grew up, folks from Badwater Texas usually are very impressed with the mountains, lakes, rivers, etc. where people from the mountain states find other "wow" things to remember. As a former Ozark American (formerly referred to as an Okie) anything with water gets my attention, hot springs, rivers, waterfalls, etc. Photo opps are everywhere, wildlife, scenery, etc.
It is a trip of a lifetime for most and one you can do over and over as many of us on the forum tend to do. The Alaska Hwy is no longer a wilderness highway, but is now a good paved secondary road through a beautiful wilderness. Drive at a speed appropriate to the road conditions and all usually will go fine.
Here is some more verbiage from my web sit,e about what to do with 2 weeks in Alaska, primarily written for the fly and rent visitors.
http://www.pajbcooper.com/two_weeks_in_alaska.htm
