In Denali everything is green-green-green through most of the summer. Beautiful but monotone. Then in late August the tundra starts to change color, as do the small number of non-evergreen trees. Shades of red, orange, yellow ... just beautiful, especially on clear days when you can see the mountains in the background.

For trees, there are some aspens near the first part of the park that are turning yellow at this same time, and the willows (small trees, not much more than bushes) also turn shades of yellow amidst the tundra. But the tundra is something else as whole mountainsides turn shades of red and orange.

As another bonus the larger animals are more active and easier to see late summer and early fall, especially bears feeding roadside in blueberry or soapberry patches, and moose and caribou, which are starting to strip the velvet off their antlers and prepare for the fall rut.

And one more bonus, the damned mosquitos are pretty much all gone.

So anyway, for Denali the absolute two best weeks are the last week in August and first week in September for fall colors.

Kenai seems to me more evergreens than deciduous trees, so I'd plan around being at Denali for the peak time and then fitting in Kenai wherever makes the most sense.

I have heard that its best to see the Kenai to take the longer tour in order to see everything.

Not sure which 'longer tour' you're referring to, but a night or two in Homer and a night or two in Seward should pretty much cover the Kenai highlights.

Congratulations on the Alaska trip, you'll have an awesome time. I did a similar trip last year.

Stayed at Park's Edge Log Cabins in Healy, which is a few miles north of Denali National Park entrance; the place was rated well on tripadvisor and priced well, so I picked it. Enjoyed my stay there, would recommend the place.

The drive to Seward is amazing. Good call on taking the longer tour to see Kenai Fjords, they have day cruises from Seward, went for the 6hr one, was well worth it.

Have received some great advice from folks on fodors, sure you will too. Have fun planning.

Ah, I guess the 'longer Kenai tour' was on the glacier tour from Seward. Sorry, I thought maybe you meant a tour of the Kenai peninsula ... got it now.