For the most flexability a car rental will give you the most. How are you getting to the Kenai River?? You need a car.
How much time do you have. You ideally need at least 5 days, with an overnight flight out.
You may want to consider also, Seward, it's a gem not to be missed, if you can add another couple days.
As for Denali Park, you will want the shuttle bus to at least Eielson, mile 66, and ideally Wonder Lake, mile 85. However, in July, you need to make a hard decision on your "bug" tolerance at Wonder Lake. It's so significant that I no longer consider it until mid August or later. Just too much of a problem for me, impacting my enjoyment (and I've used the bug nets
has the shuttle bus information.
I'm one who always prefers the McKinley summit flights, having done all the routes. It certainly is excellent.
You may want to consider one of the two purchased coupon books for savings. New ones come out in late fall.