Hong-Kong to Hokkaido

Hi,
Me and my wife want to travel from Hong-Kong to Hokkaido and return through Osaka, as we plan to stay a few days there as well before returning to HK.
What would it be the best and not so expensive way to do it? Would it be better to just take flights or maybe take the train from Hokkaido to Osaka?

We will travel from the end of June till the beginning of July, around 2 weeks. Our plan is to do a road-trip through Hokkaido for about 10 days and stay like 3-4 days in Osaka.

It will be cheaper and best to fly from Hokkaido to Osaka. Check out JAL and ANA's discount air programs: http://www.japan-guide.com/e/e2364.html

Hi,
When do you think it would be advisable to buys the flight tickets? We plan to travel late June, but right it now might be too early to book them. Should I wait until 3-2 months before the date?

Thanks

Generally speaking, ticket prices for domestic flights in Japan never decrease over time, they only increase or remain fixed, so as a general rule you should buy your tickets as soon as possible.

The main exception is with low-cost carriers, which have relatively frequent sales periods.

I have just noticed that several mainstream companies like JAL or ANA have some kind of explorer pass for the amount of 10.800yen, but I am not sure if it pays up.
Considering my situation, taking a international flight from Hong Kong to let's say Osaka and then buying a that pass to travel from Osaka to Sapporo returning to Osaka afterwards.
Does anyone has some pointers on this?

Consider Peach Aviation.

http://www.flypeach.com/pc/en

Flight fare from Hong Kong to Kansai is around 9000 yen one way and from Kansai to Sapporo is around 5000 yen. This is a LCC, so you may have to add some baggage fee or something, but I think it should still be very cheap (anagram of peach :).