Need help planning my trip

Hi, i have 3 weeks in the south island but have no idea how long to spend in each place. I was planning to stay in Mt Cook NP for 6 nights then Mt Aspiring NP for 6 nights then use the rest of the time for whatever. I will have a car. I'm not into clubs or anything like that, just love being outdoors, tramping, climbing and photography. What's the best way to get out to Fox Na Franz Josef Glacier?