A specific lens depends on your budget. The Canon lenses are going to be more expensive than the third party lenses (Sigma, Tamron, etc.). Because so much of what Disney does, is with light, indoor shows, nighttime parades, dark rides, etc, if your budget allows, I would recommend something with a constant f2.8 for everything except dark rides. If the dark rides are a "must" then I would recommend the Sigma 30mm f1.4. Personally, and I don't recommend what I'm about to write because it can become expensive, I use a 28-70 f2.8 for walkaround, 70-200 f2.8 for reach and the Sigma for dark rides and sometimes for nighttime parades. The downside to the 2.8 lenses is expense and weight. Otherwise, you can use your 18-135 for a walkaround and I think Canon makes an inexpensive 50mm f1.8. That is not the best option, but it is one that can be made to work for most situations. You will have to choose what it is you are going to shoot then make a decision. Remember cameras and lenses are like tools. If you need a screwdriver your not going to select a hammer to do the job!
Like the others, I would recommend a tripod. That is another whole controversial subject. If you are flying then I would recommend a travel tripod. A travel tripod is lightweight and compactly folds, but yet sturdy enough to hold your gear. I hang my bag on my travel tripod to make it more steady.
Good luck with your choices.