I visited Denali twice, both on Labor Day week, and lucked out on both occasions with the weather. On the second visit, I took my parents with me, and it was a real treat for them. I redeemed SkyMiles for both trips, 32K5 for the first trip (the back end was on domestic business - not my choice, that was the only availability), the second trip was 25K each for a total of 75K. Both trips are AUS to ANC, with various connection points. On both trips, it was quite easy to get award seats since the availability were good on both times.
It's better to buy bus tickets in advance, and also know which turn-around point that you want. I took the bus to Wonder Lake on day 1, and Eielson visitor center on day 2, on both trips.
The advantages of traveling during Labor Day week are the crowds are not as heavy, the tundra starts turning red, making the scenery even more spectacular, and the bears are more active due to their prep for hibernation. On one of the bus trips, we counted 8 grizzlies!
One of the posts have mentioned that you can get on/off at any spot. And yes, there is no trails, or very limited. There are trails closer to the entrance, but very limited. There was a trail near Savage River where the rangers like to take visitors on guided hike. On one of these hike, a fellow bus passenger was able to take video of a wolverine!