There are a couple of cruise boats here that will suit you very well.
The vast majority of cruises are snorkeling orientated with a couple having kayaks on board.
Pacific Sunrise is an excellent vessel and probably one of the best if not the best cruise available here. She does visit some very good snorkeling spots and has kayaks on board as well. The food is excellent and the itinerary is very good and her accommodation is the best available on a boat.
Summertime is also a very good cruise. Probably not in the same league as Sunrise but a good cruise never the less. She also has on board diving capabilities as well as kayaks and plenty of snorkeling opportunities.
There are a few bush walks here and one very big one that takes 3 days and the islands have some great walks.
I have snorkeled in both The Whitsundays and PD and much prefer it here as the islands besides adding protection from windy weather and sea swell just add to the attraction. It is a very special place.
There are a lot of choices here in cruises and the decision on which boat can make or break your experience. Skimp on the accommodation side of things and get a good cruise if the budget is a concern.
With the time you have you can have a couple of nights in Airlie, cruise for a few and maybe a couple on an island resort. South Molle and Long Island have some great walks, the resorts themselves are 3 star basic but the walks (especially South Molle) are terrific.