We took our son on his first trip at 18 months, but then again at 2 and a half and 3. So really close to your son's age.
First wanted to say don't stress on the potty training thing. My son wasn't even potty trained at 3, he actually turned 3 at Disney and wasn't trained until a month later. And except for having to carry pull ups to the parks, I would have rather had it that way over being in line and having to rush him to the potty, because at least with him when he was first trained, it was a last minute gotta go now thing.
Second, my son loves characters always have. I prepared him a little bit before by taking him places that had characters, like Chuck E Cheese, Red Robin and local mascots. I think it really helped. Also you might want to have an early on Character meal, and maybe let that be the first experience with them. I think it might be less strained. Maybe. And let me tell you our best character dining is always at O'hana for breakfast. The characters always seem to have more time to spend with us, as compared to Chef Mickey's.
Third, my son was still napping at that age, and we would schedule a nap back in the room. But let me tell you it never happened that way. We would head back to the room, lights out and all of us would lay down and he was just too excited to take a nap. We would even wait for an hour and no nap. So then we would be like well let's go back to the parks and he would fall asleep on the bus or in the stroller as soon as we got there. So I finally was like he'll sleep when he needs to sleep and I'm not going to worry about it.
Fourth, my son loved all the rides at that age. I will tell you though that sometimes it's the rides that you don't expect to scare them, that will. When my son was 3, we rode Living with the Land for the first time, and the thunderstorm at the beginning scared him. POTC didn't even scare him as much as LWTL scared him. And the only part of POTC that scared him was the fog at the beginning, but he still loved it.
Fifth, baby care centers are good even for 2/3 year olds. We have gone in there before to take a break from the heat, and my son loved it. He didn't want to leave.
Sixth, take extra clothes, especially for the water play areas. I always find it funny to see the parents telling the kids don't get too wet. I'm the parent who said go for it, I planned ahead for this. And my son loved it.
Seventh, on my son's 3rd birthday trip, I brought his birthday presents, which were all Disney related. And the presents weren't from us, they were from Mickey. And he loved it. It kept him from wanting tons of things in the park. And now that he is older, he gets his own gift cards each morning from a different character. Yes, he will soon be 7 and still believes that Mickey and the other characters some how come into our room and leave him a gift card. It's so cool.
Eighth, we still haven't done any of the waterparks, because he isn't a good swimmer. And we always hit each of the 4 main parks. At that age, his favorite parks were probably MK and HS.
Every child is different and what happens on one trip might not be the case on the next or even on the next day. So the main thing I would say is just let lose, try not to stress and enjoy the little moments. When I look back at my pictures, a lot of times, it's the pictures that I took of the little things that I love the most. Like my son running to me while waiting in line for the bus. Or him acting like a bunny when we saw a bunny at the resort. You sound like you are going to have plenty of time for those moments. Take tons of pictures. Do photopass, because I still look back on those pictures and love seeing him grow as he is holding Tinkerbell in his hand. And remember that you can never take too many pictures.
Hope you have a magical time.