Wrangell/St. Elias NP is really nice and less touristy, but is tough to add. As you said, first of all, the most direct way is to drive the Denali Highway, but most rental car companies prohibit that. Also, the same rental companies prohibit driving on the McCarthy Road. But see below for an option.
Wonder Lake is by far my favorite campground in Denali. In fact, I am going there next month for 3 nights. But I will be staying at Tek campground for the first time in September.
I would change your itinerary a bit and do this:
Aug 5: Drive from Anchorage to Seward
Take the 3pm Kenai Fjords tour this day. That will give you the entire day tomorrow to see other things in Seward
Aug 6: morning Sealife Center if that interests you (kind of an aquarium but much better - could also do this upon arrival on Aug 5), hike part or all of Harding Icefield Trail. A park ranger leads this hike on Saturdays at 9am so if you want to go in a group, you could do this. Figure back by 4pm if you start at 9am and hike to the top. Excellent hike, but not easy. 3K feet elevation gain over 4 miles one way.
After activities in Seward, drive to Girdwood or Anchorage or Wasilla for overnight
Aug 7: shop at supermarket for groceries for Denali (no food inside the park except at the park entrance and any groceries north of Wasilla are much more expensive), drive to Denali (about 5 hours from Anchorage). Stop in Talkeetna if flightseeing is of interest to you. Veterans Memorial at mile 147 is nice and there are pullouts for Mt. McKinley viewing if the Mountain is "out."
Now you have a choice. Drive to Tek and spend 3 nights at Tek campground (and buy the "Tek pass" which gives you unlimited shuttle bus rides into the park from there) or spend the first night at/near the entrance and only spend 2 nights at Wonder Lake.
The main reason I am staying at Tek in September is this same problem. By staying at the entrance on Aug 7, you get one bus ride into the park on Aug 8, a full day of bus/hikes on Aug 9 and one bus ride out of the park on Aug 10. By staying at Tek all 3 nights, you can start early on Aug 8 and Aug 9 and get two full days of buses and hiking inside the park. If you book Tek for the night of Aug 10, you can leave your car at Tek while you spend yet another day in the park (otherwise campground check out time is 11am, so it would not give you any time in the park that day). Then later on Aug 10 you can drive to the entrance.
It is a tough choice. I really enjoy Wonder Lake, but you basically lose at least a half day in the park by spending the first night at the entrance and two at WL.
If you spent the night of Aug 10 in Denali, then I would go with your plan to stay at the entrance on Aug 7th.
Or, if you want to try and get to Wrangell/St. Elias NP, and you stay at Tek, you can get out of Tek earlier than Wonder Lake (absolute earliest to get from Wonder Lake to the entrance is depart WL at 6.30am, arrive the entrance about noon). With Tek, you can depart anytime in the morning, figure 60-90 minute drive to the entrance. Note there is a 3-night minimum stay at Tek and you can't move your car once you arrive at the Tek CG until you exit the park.
Are you interested in hiking on a glacier? How about this?
Aug 10 drive from Denali to Wasilla and along the Glenn Highway to Matanuska Glacier. Do an afternoon glacier hike (Mica or Nova guides), overnight in that area (several campgrounds and/or cabins).
Aug 11 drive to Chitina, take Wrangell Mountain Air to McCarthy at 2.45pm or 5.45pm (
http://www.wrangellmountainair.com/g..._to_alaska.htm).
Aug 12 morning Root Glacier hike, afternoon Kennecott Mill tour or if you want a strenuous hike, hike to Jumbo or Bonanza mine (all day hike). Can tent camp near McCarthy or also not too far from the glacier in Kennicott.
Aug 13 morning flight back to Chitina, drive to Anchorage for flight home.
You can also stay in McCarthy, and pretty reasonable if you use a Toursaver coupon (toursaver.com, which also has Kenai Fjords tour coupons from Seward, Root Glacier hike coupons, among others).
If two glacier hikes are not in the cards, I would do the Root Glacier one. Then on Aug 10 you have time to enjoy the Matsu Valley (stop in Talkeetna for flightseeing if it was canceled on Aug 7), Iditarod HQ in Wasilla, Independence Mine area, Palmer, Muskox Farm and Reindeer Farm in Palmer, etc and stay as mentioned above for Aug 10. This is all on the way to Chitina via the Glenn Highway.
>>I apologize for the somewhat rambling nature of my query, our thoughts on the itinerary are not fully formed yet and you can see that reflected in the post.
You don't know a rambler until you read through my reply!