Bring a stroller of your own. We were so happy we brought our stroller as it reclined and DD could sleep in it. We bought a cheap umbrella one that reclined jsut for the trip. The ones you rent don't recline well.
The changing stations at the parks are great for changing, feeding etc. They are QUIET and it is a great break.
Bring snacks and bottled H2O. Our DR. said that the H2O in Florida has a lot of sulpher in it and the baby probably would not drink it. We brought H2O with us into the park each day. Also bring sunscreen and snacks.
If your little one is still using bottles remember to bring dish detergent. I like the towelettes that already have soap on them to wash out the bottles and nipples back at the hotel.
Link favorite toys to the stroller with those plastic links so you are not constantly picking up dropped toys all around the parks.
Put some kind of identifying mark on your stroller ( a pompom on the handle etc). While you are on the rides they sometimes move your stroller. Our stroller was navy blue and so were 100s of others.
If you are staying on Disney property you can mail yourself a box. We mailed down a few toys, bottle bags, wipes, diapers and a few other things. It saved us space on the plane and DD was happy to have something to play with at the hotel.
If this is the first time you will be using sunscreen on the little one, try it before you leave. DD got hives from the sunscreen and we had to call DR from our cell phones at the park to find our how much benydryl to give her

She looked terrible. Ditto for bug spray...try it before you leave.
If you are flying make sure you have bottle, cup or pacifier for take off and landing so babys ears dont hurt.
There are some great links on these boards for activities for your 5 year old..coloring pages, etc. There are also some trivia and questions you could help him find the answers to at the parks. There is also a great journal that you and he could do togeather so he has some great memories. We gave our other DD a disposable camera for the trip so she could take her own pictures...worked great.
Remember a great big fat pen (you can buy them in the parks) for autographs. It is easier for the characters. I hope some of these help you out.You are going to have a great time. The best part will be watching the childrens faces........magical.