Tablets are a much better vacation-computer than a laptop. They provide the ability to look up directions, itineraries, and information about your travel destination without being bulky to take along with you. The few-moving-parts design makes them somewhat more robust to being taken places like the beach than a laptop would be (though do be careful even so). Plus they can double as a GPS navigation system.
I do all this with my HTC Incredible, and it fits in my pocket.![]()
True, phones can serve those purposes as well, but there is something to be said for the larger screen. Especially when a given website doesn't happen to have a mobile version.

I do wish though that my career afforded me the ability to go offline for extended periods of time. As it is, I'm expected to be available most of the day (even when I'm not at work sometimes) and so the iPhone has been a boon in assisting that particular requirement of my job. I'd much [rather adopt a philosophy more in line with what Dennis was describing, but for now it's just not in the cards for me.