Emmeline's post on IEP vs. 504 is very good. An IEP is alterations to the curriculum because your child has a disability that makes it impossible for them to achieve the required benchmarks in the curriculum. A 504 plan is accommodations to the learning environment to allow your child to participate in the same curriculum as everyone else.
My son was diagnosed with SPD and visual-motor integration issues by the school OT during 2nd grade, when his sensory issues (that he had all along) rose to the level where they began to interfere with the educational process.
His 504 plan included weekly OT sessions for his handwriting and visual integration issues, weekly OT with techniques to help his proprioceptive issues and need for sensory input.
Other things on it were that the teacher would allow him to do his work standing next to his desk, or kneeling on his chair with one knee while standing on the other leg. She also gave him regular "heavy work" around the classroom and actually re-instituted some exercises she used to do but had stopped, where between lessons they'd all stand up and do certain head and arm movements. She also worked out a signal with him so that when he was doing something distracting to the other kids, like jiggling his knees so the table shook, she'd signal him to make him aware of it (he was often unaware he was doing stuff with his feet and legs like that) so he could stop himself without her calling him out and making a big deal about it in front of his peers, which meant a lot to him.