There are plenty of rides for 2.5 year-olds, especially in the MK. Pooh, the Carousel, It's A Small World, Peter Pan, the Teacups and Jungle Cruise are all great for kids that age. The People Mover in Tomorrowland is nice for a relaxing break, as is the train. Depending on your child, Pirates of the Caribbean is good, but it is dark at the beginning. Also, don't miss Mickey's Philharmagic. My daughter was 3 her first time, and this was by far her favorite thing in any of the parks. If your child is familiar with the more recent movies like Little Mermaid, Aladdin and Beauty and the Beast, this is a must. It is really funny and not scary. You'll love watching your child try to grasp the 3D objects. Plus, it has Fast Pass.
That's another thing when traveling with your toddler...pick the amount of time you think your child can wait (if anything like my children at that age, not very long!), and skip any rides with longer wait-times. My children have never been on Dumbo for this reason. Pooh, Peter Pan, and Jungle Cruise all have Fast Pass.
Mickey's Toontown has a nice play area for toddlers...no bigger kids allowed. It's a good place for them to run around, climb on pint-sized slides and take a break from the crowds and lines.
As far as time is concerned, it really depends on how crowded the park is when you are there. In April, the first day we went to MK, we were able to go on all the rides we wanted to go on by 2 PM (arrived at 9). This included lunch. The second day, the park was so crowded we ended up leaving because we couldn't get on any rides without waiting at least 45 minutes...far exceeding my children's patience level. I would plan at least one full day for MK.
Also, if your child likes meeting the characters, I would recommend going over to Epcot. All the major characters (Mickey, Minnie, Pluto, Goofy, Donald) as well as some of the others do photo sessions at various times. You can ask where the characters will be and when. In my experience, it is far less crowded meeting the characters at Epcot than at MK. We waited 20 minutes for Mickey in MK, while my children were able to meet Minnie, Pluto and Goofy all in less than 10 minutes at Epcot.
Have fun!