If you want a cool horseriding trip, one
outfit does trail rides into the Haleakala crater. Wife and I have done this twice and it was great each time -- the views are incredible, and they take you right down to the crater floor, where you can see the silverswords, rock formations, etc.
As for snorkeling, I'm not sure about catamaran trips, but there is fantastic snorkeling around the base of Black Rock right on Kaanapali Beach. If you go early it's usually no more crowded than the main "off-shore" snorkeling sites.