we went in January with our 3 1/2 year old. We had a blast.
He did every ride that he was tall enough for (40 inches).
Some things that helped were-
snacks- I brought a bunch into the park. A water bottle with a built in filter.
A good stroller- we brought our chicco umbrella stroller- it was nice because it reclined. I had him nap in it every day. We would recline it, put a blanket over the front and find a place where we could just walk in a big circle (areound the hub at MK, around the imagination pavilion at epcot, and the streets of America at DHS). I don't know what we would have done if he hadn't napped. We got there early, had him nap and stayed late. it worked well.
Lots of character M&G's. They were all a hit! Even the princesses. Ariel and Belle were his favorites. He was amazed when Ariel gave him a kiss on his cheek and still talks about it today.
Read his signs. IF he needs a break, take one, and give him a snack. Or put him in the stroller and let him sit while you walk. I am so thankful we had the stroller.
For the lines- I found a bubble gun thing on the first day we were there . It was awesome. The refill bottles were like .99 cents and it kept him entertained for all the lines. It even entertained the kids around him too. He had a blast blowing all the bubbles and then the kids were all popping them. I also found this light up buzz light year thing that you push the button and it spins around. THat kept him very entertained during long lines as well.
For Fp's we were there the weekend where things were turning over form legacy to FP+ for offsite guests. So we used FP+ at MK and legacy at DHS and Epcot. We used them for - ETWB, BTMRR, and Peter Pan. Soarin and Malestrom (pre- tiering), and Star Tours, Toy Story and Voyage of the LIttle mermaid at DHS.
Hope that helps!