If I want to travel both islands what is the ideal number of days I should go? Also, what are some of top places you recommend I visit?
I just spent the past five months living in New Zealand for a study abroad program and it was amazing! I was based in Auckland on the north island and was able to take short trips around the island on the weekends and then spent two weeks on the South island for fall break.
If you're flying all the way to New Zealand for a real adventure I would recommend spending AT LEAST three weeks between the two islands. Our South island trip was just long enough for us to see all of our highlights, but we still didn't get to see everything. Having lived on the North island, I would go back and spend more of my time on the South island because there is more to see.
Keep in mind, there is no mass public transportation in NZ so you'll have to rent a car and drive everywhere you go and gas is pretty expensive. If you like camping, a small economical car will do fine to get you from DOC site to DOC site where you can camp for a small fee. Or, if you want to have a more leisurely trip but don't want to worry about having to find a place to stay in the middle of nowhere, you can rent a little camper-van and use that as your accommodation. Airbnb is also pretty good in most places and we used that a few nights during our two-week trip when we got tired of sleeping on the ground. I also had some friends that successfully hitchhiked all over the South island if that is something you're brave enough to try.
Some highlights from the South Island:
Milford Sound - if you don't see this then you've wasted a NZ trip
Roy's Peak trail in Wanaka - challenging hike but THE most amazing view I've ever seen
Queenstown - cute ski town with the famous Fergburger that you have to eat at
Franz Josef Glacier - if you can hike Robert's Point Track, there is also Fox Glacier but it isn't as impressive
I wrote a blog while I was traveling that has more trip details and pictures if you want to check it out. www.kirstengoeskiwi.wordpress.com
I hope some of this is helpful. Enjoy your trip! New Zealand is unbelievable!