The traditional three alternatives are Maui, Hawaii ("the Big Island"), and
Kauai.  Molakai and Lanai receive far fewer tourists and have no direct
flights from the mainland U.S.  Each of the islands has its own merits.
Maui is the most developed and most expensive, but also has the best dining
and most tourist attractions, including snorkeling trips over to Molokini
Crater.  The Big Island is big.  It also has plenty of tourist attractions,
but you'll be driving a lot further to get to them.  It's cheaper than Maui
and more laid back and snorkeling there is excellent due to the water
clarity - as a relatively new island, there's very little dirt to cause
run-off problems (this pertains to the dry west coast, not the wet Hilo
side).  Kauai is the smallest of the three, and the oldest, so the water
might be a bit murkier but that's not really an issue at snorkel depths.

If money is no object, and crowds don't scare you, I'd opt for Maui as
having the greatest range of family activities.  On the other hand, if you
want a more laid-back island experience, the Big Island or Kauai might be
better.  You won't go wrong with any of them and, if you have the time, you
can always check out more than one island.  Just don't make the mistake of
cramming too many islands into too little time, or you risk spending half
your vacation at the airport.