Thanks for reaching out. Your itinerary sounds great!
You could certainly spend a few nights using Portrush as your base, as it is very central to a lot of activities and things to do on the Causeway Coas
t. In terms of Donegal, I would recommend checking out this page
which is a good starting point for what may interest you. I also think this trip idea
would be a good fit, as you have mentioned hiking and it begins in Greencastle which is an hour from Portrush.
I would recommend the Donegal Bay Waterbus
for boat tours.
Let me know if you have any other questions, and I would be glad to help.