Without knowing how you plan your days (do you do rope drop, take a midday break, stay for evening shows), it’s tough to answer the “will it fill a day or not” question. Part of the answer would also depend on if you’re a regular Disney World visitor, too, in which case some enjoyable attractions (Soarin, Toy Story Midway Mania, and to a lesser extend, the Little Mermaid ride, might not be as special, since they essentially the same as the Florida versions).
Your interest in/tolerance for fast-moving rides or those with significant drops will also make a big difference. Disneyland is full of many rides that are accessible for all guests, while many of DCA’s best attractions are more in the “thrill ride” category. I visited over the summer and couldn’t ride Guardians of the Galaxy: Mission Breakout enough, since it features a variety of different songs, each with slightly different shows, and I enjoyed all of them. However, two of the three other people I was traveling with wouldn’t ride this at all.
The same could be true for the Incredicoaster, which is a very good ride, but only for folks who enjoy roller coasters. On the other hand, Radiator Springs Racers is a great ride that’s accessible to just about anyone. Since part of that ride takes place outside, I’d also recommend riding in the morning and evening. Actually, all of Cars Land at sunset and just after dusk is a different experience than in the daylight, so if you can time you FastPasses right, it’s worth riding twice.
World of Color is a great show, too. Pick up a FastPass for that in the morning, since almost all of the viewing areas are reserved that way and that FastPass doesn’t prevent you from reserving a FP for something else. Just beware that if the breeze is blowing the wrong way that you might find yourself getting rained on a bit. All of the water from those fountains has to land somewhere.