The one-way car rental price sounds about right. But I would never recommend going to Seward for just a couple of hours. Instead, I would book a Prince William Sound cruise out of Whittier. They will get much closer to glaciers on this trip than on their cruise ship. Cost +-$150/person but there are some 2-for-1 coupons with toursaver or northern lights coupon book. They can then take the train to Anchorage that evening.
On the day of arrival into Denali, they can see the visitor center exhibits and film, the sleddog demo, the film at the wilderness access center, do some light hiking near the entrance (with or without a ranger), or take the free shuttle bus to Savage River (15 miles into the park).
The shuttle bus in Denali is definitely the way to go. They go much further into the park than the tour buses and cost about 1/3 of the price. Also, with the shuttle buses they can get off and on at will. With the tour buses, they stay on the same bus all day. Be sure to book the shuttle bus in advance and you might want to call Denali Princess hotel to find out when their earliest shuttle runs into the park. They must bring all their own food and water for this trip - so probably buy something in Anchorage the day before the train or there are limited options in "Glitter Gulch" near the Denali Princess. Water bottles can be refilled at Eielson Visitor Center, which is the minimum distance they should book the shuttle to. If they book the shuttle to Wonder Lake, they must be prepared for mosquitoes - high concentration DEET and/or mosquito headnets. They are really bad through early to mid August but only near Wonder Lake.
Lots to do in Anchorage. Check out the recent thread in this forum about that. I would recommend the Native Heritage Center for one.
What they didn't see the day of arrival they can see on July 10th before the train. There is luggage storage near the visitor center so they can check out and take their luggage with them, thus avoiding backtracking later in the day.
In Fairbanks, Riverboat Discovery, Gold Mine or Gold Dredge tours, U of AK museum is excellent, maybe see the pipeline, maybe go to North Pole, about 15 miles max from Fairbanks (a public bus goes there if they are not renting a car).
I would get to the airport 2 hours in advance in Fairbanks in the summer - cruisetour passengers visit Alaska in droves and lines can be excruciatingly long.
John