I never took our Kelty to WDW with us for this reason...it was too bulky to bring on rides, yet not something leave laying around. I ended up ebaying it and getting a Baby Trekker which is cloth, almost as comfortable as our Kelty pack was and can be packed up fairly small (you roll it up into something a bit smaller than a soccer ball). There is also a carrier called the Ergo that is very similar to the Baby Trekker, but a bit nicer looking imo. Both of these carriers can be used on either the front or the back and hold the baby directly against you....sort of like a Snugli, but not uncomfortable like a Snugli. You could also try a sling carrier...but those are not as supportive as the backpack carriers.
There are a fair number of rides that you can all enjoy that may not have room for the backpack. I'm not sure how you would handle that as most rides do not load and unload in the same place. Maybe you could park your backpack with the strollers? I don't know if I'd be comfortable doing that...but it might work. Generally we just use a sling and stroller....though if I know I'm going to be doing a lot of carrying I use the Trekker or a different cloth carrier I have.
Good luck
