Personally, I'd prefer to bring an inexpensive umbrella stroller to renting...at the airport (at least with Southwest) you can bring it right up to the plane, and they will do a gate check. You also get to board in the preboard group. Make sure you have ID for the baby if you plan to fly as a lap chilld! Umbrella strollers are lightweight, inexpensive, and you can use them to help carry stuff. Personally, I like mine with a basket under it.
I'm also of the mindset that the less junk you carry around with you all day, the better. I don't subscribe to the idea of bringing a lot of toys, blankets, etc. with me everywhere I go. My daughter doesn't expect them and she finds great ways to amuse herself, that don't involve me lugging lots of junk. She is also very observant of her surroundings and people. Disney has really cool things to see everywhere. If there is one thing an infant can get out of Disney, it is the amazing sensory experience of being at Disney! If your child is primarily going to spend time looking at toys from home - why bother bringing him to a new place?
DO bring sunscreen, a few small comfort-food snacks, water, a sweater/second layer.
Are you nursing? Using bottles? I use a combo - so I can offer tips on both.
Have you heard of baby-swapping? This is a little known practice. If your group wants to go on a ride like Space mountain, one person gets in line (or the group minus one), and the child-sitting person can ask for a baby swap pass. Then when person one gets off the ride, person two (along with up to 3 people) gets to go in the Fast pass line! This can be a little time consuming if you are waiting in a regular line though and the parks are busy.
One of the great things about Disney is that there are actually a number of rides that you can take a little one on. One of the best are the shows. Try to pace yourself so that you are going to a "Show" type attraction, every so often. My 6 month old LOVED the Lion King show at AK, or at least the first half, as it was also a good chance to nap.
It also makes sense to expose your young one to Disney characters and music before you go. My daughter already knew who Pooh was, so she wasn't scared when she saw him at the park, although this might not work with some kids. (she also knew the Lion King music - so it was new and familiar at the same time)
With a little one, it is great to have more than two adults in your party - at least that's what we thought.
If you can afford it, it makes a HUGE difference to stay on property at a centrally located deluxe resort. Time is money. If you are at the Poly, say, it is very easy to return to the hotel mid day. It you are off property, it almost isn't worth it to break during the day. The Swan is a great property also. See if you qualify for any of the discounted rates. It is a great location!