I would try various national chains for one-way rentals (Avis, Hertz, Budget, etc). But some of those don't allow their cars on the ferry.
Also, ensure the company you rent from allows their vehicles in Canada. Avis (the only company with an outpost in Haines) does, but if you end up driving to Haines and doing a loop to return to ANC, this is something to be aware of. For example, Dollar and Thrifty do not allow their cars out of the state of Alaska.
I've driven the Alaska Highway three times and would say that nothing between Glennallen and Jasper National Park in Alberta sticks well in my memory (except the signpost forest in Watson Lake, but that's hardly worth driving thousands of miles for, or the long northern route up to Dawson City, which is massively out of the way). On the limited time you have, I'd suggest skipping the drive to Haines; see Haines by ferry but don't bother driving there.