Just about everything that's appropriate for your 3-year-old will be appropriate for your baby. Your baby can go with you on any attraction for which there is no height limit. For a complete chart of attractions with height limits, click
here. If your son is of average height, the only attraction that he is likely to be able to go on that your daughter will not is Goofy's Barnstormer which is located in Toontown in the Magic Kingdom (check the chart to be sure). For more tips about traveling to WDW with younger kids, take a look at the "
Little Ones" section of the WDW website.
I think every Mom on this panel has been to WDW with infants and toddlers. I first took my twins to WDW when they were nine months old. Of course that doesn't count the trip they took when I was pregnant with them :-)
My experience when my girls were that young was that they didn't particularly care about the attractions and rides more than anything else about the park. They were happy just for the experience. They loved looking at the colors and hearing the music, but they didn't so much care about the fact that they were on a particular ride. My girls favorite part of the trip when they were that small was meeting the characters. Fuzzy fur and big noses were endlessly fascinating.
As long as your daughter is well-rested and you seem happy, then I'm sure she'll have a great time at WDW.