Definitely agree with Jackal - if the weather's great, do the flight. If not, the cruise is fun enough.

The drive to Whittier is gorgeous, and the tunnel is interesting. Whittier itself is, well to put it nicely, a little bland.

The Alaska Railroad does run to Whittier in time for some of the cruises - might see if that's an option to combine even more activities together. If you wear a hat, you may not by the time the cruise is over - the boat moves very quickly, and if it weren't for a fast-acting German guy at the back end of the boat, I would have been hatless. So hold on to your hat.

By mid-May the days should be getting pretty long - you may have more time than you think to do/see things. I would also recommend Earthquake Park and possibly pulling into the back parking lot of the Alaska Motor Vehicle Department by the airport - great place to sit and watch the floatplanes takeoff and land.

If you do end up doing the floatplane trip, please report back about the experience - it's the one thing I haven't done in Alaska yet, though a similar trip in New Zealand was a lot of fun!

Have a great and safe trip!

Sean