You must get a fastpass to watch World of Color. That may make seeing Fantasmic a bit easier to do on a late arrival day.
If you choose to watch Fantasmic, when to start waiting depends on what kind of view you want and whether you are watching the first or second show. If you want front and center sitting, some people start waiting as much as two to three hours before the first show starts depending on the time of year. You can often still find good standing viewing though even for the first show 20-40 minutes before it starts.
If you want to see World of Color, you can pre-order a picnic lunch in order to get a WoC fastpass for the first showing or you can go to the fastpass machines when you arrive in the afternoon and get a pass that will probably be good for the second show or you can do one of the sit-down dining options for a premium viewing fastpass. Don't know if you want to spend extra money for the dining options though. WoC requires a lot of waiting for a good view. You need to arrive at least an hour in advance and would probably do better to be even earlier.
As for what to do on a Sunday afternoon...
Big Thunder often still has fastpasses available late in the day for use a short time later. Small World and Buzz Lightyear often have modest waits in the afternoons. Shows like the Tiki Room or the parade are good late afternoon activities.
In DCA, Screamin' sometimes has a decent fastpass return time, but those do start to disappear since Screamin' will close early for WoC. We like to ride Toy Story since the line builds quickly in the morning and stays long the rest of the day anyway. The Animation Studio, one of the 3D movies, the parade, or some of the shows on Buena Vista Street are all good ways to pass some time.
