The way to Cape Trib is great. We loved staying the Red Mill in Daintree (lovely small village). You can drive up and down to Cape Trib and do a variety of places from there.

RE: the whitsundays. I have not done them all but really the only truly exceptional one is whiteheaven for which you'll need a tent (and permit well in advance). The resorts on the others did not look truly exceptional (but good enough to have a great time). You'll need to fly to Hamilton from Cairns else the drive from Cairns is very long. If you are driving down consider the Sanctuary Retreat in Mission Beach as a 1 or 2 day stopover (very relaxing place with good views and nice lux huts). You may be lucky enough to see some Cassovaries in the wild...

Also since you're a bit short on time, consider avoiding travelling down at all and spending time on Fitzroy Island (and perhaps Green Island as well). It is really a great place.

Whatever you do and wherever you go, if you've never been, you'll have a great time!

If you do not dive consider giving it a go.