Naval combat.

[Geoff]

I am thinking ships will mostly be for travel, but I like to plan ahead. If trips are too short that does unbalance 4E and Essentials classes, but is that as big a concern as I'm thinking?

You mean like an accidental 15min workday? There are only a handful of Essentials classes that lack dailies like that, and even if they're in play, they won't be that badly overshadowed unless the other characters figure out that encounters are going to be few and far between on trips, and waste dailies going nova a lot. If it does look like it's going to happen, you could add a cool shipboard power or two that's slanted in favor of those classes, like ship-board weapons (with a daily power) that only the Slayer is 'proficient' in or something.

It's also less of a problem, in general, because the kind of player that might be put off by that kind of imbalance can always take the 4e version of the class instead of the Essentials - none of the no-daily Essentials classes are that unique, a Slayer becomes a Great Weapon Fighter, a Knight a guardian one, a Thief an Artful Dodger Rogue.