Where should I take my family on vacation?

We've been telling the kids for two years now that we would go to Hawaii this summer.  We have tentatively booked a condo, but I want to find out what the best places are to take the kids.  We have a 11 year old boy and a 5 year old girl.  From what we've heard, we should go to Maui; much more to see/do.  And yet, Kaui is beautiful too.  I don't want HOARDS of crowds, but I also don't want (well, I do, but the kids don't) total serenity.  I'd like to be close to the ocean and to a good beach (not on the rocks), so we don't have to drive to a beach.  I know I'm not asking much, am I?  LOL

Any suggestions/advice/tips are greatly appreciated!  

Have a great day!

Michele