Travel by plane shouldn't be a problem until later in pregnancy.
For rides, I'm sure there is a precise list somewhere, but my doc told me to avoid the rollercoasters, Splash Mountain, motion simulators. Was fine with the kiddie rides.
Walking -- ugh! I tried to do to much, too fast on our trip (with ALL of DH's extended family). Ended up at Sand Lake hospital with spotting. Turned out everything was fine (thank god), but the doctor said they see several pregnant women a week with the same thing -- do too much and wear their bodies out. I spent two days worrying before going to the hospital, a day at the hospital, and a day with full rest before I was able to get back to WDW (on a much slower schedule). Oh, and I was in A-1 primo fitness/health before the trip. Now that my daughter is 1 year old, I can look back and see what an idiot I was not to relax and enjoy the trip instead of trying to keep up with people who were NOT PREGNANT.
So, my advice is take it slow. Take lots of breaks. Drink lots of water. Sit as often as possible. Do not be the one who "Does it all" -- let others carry stuff and do everything that needs to be done. See this as an opportunity to "watch the shows" more than "ride the rides".
Enjoy your trip!