Actually there are plenty of rides that your 5-month-old can go on with you. You can take your baby with you on any ride for which there is no height requirement.
If you look at this page of travel tips for people with small children found
HERE,
you will see a button on the right hand side of the page that says
"Height Requirements." Clicking on this link will bring you to handy
pdf that you can download. The pdf lists the attractions in all four
theme parks and both water parks that have height requirements. If an
attraction is not listed on this document it means that there is no
height requirement and any age child can enjoy the ride. The height requirements are also listed on all the maps at the parks and on signage in front of the various attractions.
In addition to this, there are shows and other entertainment in all four theme parks. The music in the Finding Nemo show at the Animal Kingdom is wonderful, as is the Beauty and the Beast show at Disney's Hollywood Studios. Your baby is welcome to view these with you.
When my children were babies, they also loved seeing the characters. Your infant won't know he or she is meeting Mickey Mouse, but will be stimulated by the soft fur and bright colors. You could see characters at a character greeting zone or at a character dining experience.
I hope the whole family has a very special time at Walt Disney World.