Took DS for first time at 18mos-he had a ball. He remebers his second trip at 2 1/2 but not the first. I second the advice about trying to stick to his regular schedule and keep things as normal as possible. My two DS are used to getting up at 6 am every day, so it worked out well. We hit the parks early, came "home" for his regular nap, and played the evenings by ear. Be sure to pack a bag w/sippies/snacks/etc, also items for waiting in line (ie bubbles). Little ones find their own magical moments and you just work with it. For us, one was getting off the boat at MGM. Usually we'd be rushing to get to the gate. NOT.... my DS saw the ducks and the rest was history. We were there for quite some time watching/feeding the ducks. It was really great. Also, in AK he loved the petting zoo - something we never would have done sans kids.
Something else to think about......we splurged and stayed at BCV. It was so convenient having a separate bedroom, so he could take his nap. Also, at night we were able to stay up later after he went to bed. Having the kitchen/fridge was a lifesaver. We ate breakfast in the room some days, or at least gave him something to hold him over. (Unfortunately, we can't afford it this time around for DS 2 's first trip). Though, I love the POP!! Have a wonderful trip........