If you're running Expedition Everest Challenge, I'd recommend sticking around for the after party and see what you think; it's already included in your race registration, so might as well get your money's worth!
Our only post race party experience was last years Tower of Terror 10-miler. Aside from being tired, we had a good time taking our pictures in the parks with our medals while still wearing our race costumes. A picture of our ToT medal on the ToT ride was great! If EEC is similar, they'll have several rides open that you can enjoy as well as character photo opportunities throughout the park. There will probably also be a DJ there with some dancing and drink carts available for those of legal age.
Most people don't get to enjoy Animal Kingdom after dark, so that's one of the immediate highlights we see of sticking around for the after party for EEC.