I agree with most of the above
I disagree about the sling. At 18 mos ergonomically it just isn't great to have all the weight on one shoulder, especially if you aren't used to it. I do have a ring sling that I still use with DS (
www.zolowear.com ), but I've been using it for a long time, so I've built up the strength for it. Even still I can't use it for long without it aggravating my pinched nerve. It is nice for quick in and outs, though, so if you are up for two carriers I would get a sling as a second.
I love my Babyhawk, but I don't find Mei Tais practical for a Disney sort of outing. The ties aren't always convenient.
I am bringing a Beco to Disney tomorrow. It's great because it can be either front or back carrying. You can also roll it up and wear it like an ugly fanny pack without your child in it (fashionable, no, but nice to have it out of the way when your child will be in and out).
Similar to the Beco is the Ergo, which is a bit more "mainstream." I didn't like it as much because it is bulkier and the body is shorter, which isn't good for my DS who likes to arch his back and do back flips.
There is also the Yamo (I believe that is how it is spelled) and the Patapum which are similar to the Beco and Ergo, but I have no experience with them.
Having a good carrier, even if you are using a stroller, is great. There is so much time in lines and wandering through tight places that it is nice to have an easy way to carry a baby or toddler.