I used to be a wedding planner and honestly anything inedible does not go over well regardless of how "cool" it is unless you can afford to give out iPads.
Luggage tags come to mind for a travel themed wedding but I am truly not a fan of knick knacks. Travel tags can be awesome when done right. Especially if you can double them as escort cards! I would totally take that and keep it for long time! Plus you can just store it with your luggage and it won't take any unnecessary space!
Still, I suggest edibles. How about sugar cookies decorated to look like steamer trunks, luggage tags, old plane tickets, cruise ships, etc.?
I'm going to forgo favors altogether but someone did suggest that we do a card at each seat saying that in lieu of favors we made a donation to X charity. (We also skipped the registry and suggested an optional nominal donation to X charity instead.) So that's an idea.
The best suggestion I have was taken from a Quaker tradition that made modest donations in guests' names to really great charitable causes. It fells really genuine, kept with the theme of the shower, and simply donate the amount you would spend on knick-knacks or goodies.