Kids of that age can be really easy to travel with, if they are already used to sitting in a car for a while. We traveled quite a lot with our son when he was between 1-2 (well, we still travel a lot with him, and he's seven now). It's amazing how little it can take to keep them entertained at that age.
+1 on the bag of new toys idea. We've done that often. They don't have to be big or expensive toys. Our son would be super-excited about a new Matchbox/Hot Wheels car and would be good for a couple of days with it. He liked playing in the dirt a lot, so we'd get him Matchbox trucks, bulldozers, etc., which did double duty. Pretty much anywhere we'd stop he'd have dirt to play in. Of course, when camping, he looked like a giant dirt pile and we had to wash him down a lot, but he had fun.
We've never been huge fans of TV as a distraction for our son, but for long trips it's probably worth investing in a portable DVD player (or iPad or whatever) for those times when you have a really long drive ahead of you that you can't break up.
Toddlers want to move, though, so be sure to plan in lots of short breaks. Even five minutes of running around can work miracles.
Don't have your expectations too high for him (her?) being excited about things you think he should be excited about. You'll be "oohing" over the bald eagle overhead, and he'll be enamored with a rock. So it goes. Go with the flow; if he's having fun, you are having fun. As somebody else said, you are laying the groundwork now for him being a good traveler later.
We've driven to Alaska twice, both time in the 90s. Although some of the info is probably dated, you may get some good links on my website at
http://www.brianwasson.com/trips/alaska/index.htm. As far as Alaska/Yukon goes, Dawson City is a neat place to visit, although pretty small. Tour the Parks Canada gold dredge site outside of town. Pretty neat. Drive up "Midnight Dome," just outside of town, which offers a full panoramic view of the town and rivers. There is a free ferry across the Yukon River there, which takes you over to the "Top of the World Highway" to Tok, Alaska, via Chicken, Alaska. If you are going early in the season before that road is graded, beware that it can be rough. Just across from Dawson there used to be a nice provincial campground. I assume it's still there. Also, there are plenty of places where you can free camp along the road as you head toward Alaska, including a BLM site or two.
If you are in salmon spawning season, you'll all get a kick out of seeing streams choked with them. Plenty of places to see this, although in the touristy areas you'll find streams that are probably more toddler-friendly with boardwalks and whatnot (e.g., in Valdez, just south of Anchorage, etc.). Another nice campground near Valdez is a bit north of town, up Thompson Pass, called Blueberry Lake State Rec Site. Although, it's often full. The other one that was recommended in Valdez, down the side road that leads to the Alyeska terminal, has a great view but is basically just a parking lot.
In Denali NP, the free dog-sled demonstration is always a hit with the kids (and adults). Univ. of Alaska Fairbanks has a museum that may have some fun stuff for all of you, too.
Honestly, in Alaska basically just do stuff that interests you, and build in some things that may be fun for your son, too. It's not like he will remember any of it. And, like I said, at that age rocks are just as interesting as world-class scenery and jaw-dropping wildlife.
Have fun!
I really think that marine ferry to valdez sounds great. It'll mix things up instead of driving back on the same road. We're really just thinking of hitting Homer because of the possibility of doing some surfing so maybe the passenger ferry to Seldovia would be possibility. I just updated my 2010 milepost to the new 2012 that just came out 
Katchemak Bay and Ressurection Bays on the Kenai Penninsula are not known for big waves - unless they are caused by a seismic event. You need a town with direct exposure to the Gulf of Alaska -like Yakutat or Kodiak for big waves - unless you are windsurfing (Katchemak Bay on a windy day) or bore tide paddle board surfing on Turnagain Arm (check the tide charts): 
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