Although Inter-island flights are still relatively inexpensive, it is becoming increasingly more difficult to simply "pop" over to another island without prior planning.
There was a lengthy aritcal in the HNL advertiser over the weekend on how much more difficult this is to do now. With the recent airline reorganization of the Hawaiian airlines, there are also fewer nonstop flights between the islands so the process takes longer as most flights between "neighbor" islands now mostly connect through HNL. I guess I just want to emphasise that you should now plan ahead!