One of the advantages I've found in having an AP and being lucky enough to visit often because of geography is the chance to "do nothing". I've spent a lot of time doing that, and by that I mean anything other than rides. Since you'll be at the resort for a number of days, you'll have time for those as well, but otherwise, here are a few suggestions, of course, depending on the interests of you and your nephew.
1. Take time to look at the theming and architecture and details all over the resort. Each location has magic little touches that most people don't have time to look at and/or won't notice as they're rushing from ride to ride. Main Street, Toontown, Hollywood Backlot, Paradise Pier area and Carsland are specifically good examples of that. (Do you know about some of the hidden Mickeys in Carsland? There's specifically one that requires looking up when you're near Stanley's statue.) There's a walkway on the side of Sleeping Beauty Castle that runs from in between two of the toy shops in Fantasyland to where Fantasy Faire is now. In that walkway is a window with some nice decorations. Take a raft over to Tom Sawyer Island and explore. There is no concession stand there, so you could even buy a snack on the mainland and then take it with you. Have a seat on some rocks or any of the benches, and you'll have a relatively quiet place to enjoy your snacks, with a beautiful view too.
2. Take in the various shows available around the resort, like the Soundsational parade, Mickey and the Magical Map, the storytellings for Tangled and Beauty & the Beast at Fantasy Faire, the Dapper Dans and various other street musicians available. I like the Red Car News Boys at DCA, and some people like Five and Dime as well.
3. Attend some of the character meet and greets available throughout the resort. Fantasy Faire offers the opportunity to meet some princesses, and Mickey and Minnie can be found in their houses in Toontown. The Jedi Training Academy is also a lot of fun - I don't remember if your nephew might be above the eligibility age to participate, but it's still a lot of fun to watch.
4. Go into the Animation Building at DCA and sit in the lobby for a while, enjoying the movie clips being shown there. You can also do the drawing class or meet Crush (it can be hit and miss depending on Crush's mood and the interaction with the audience but interesting and fun) or go to the sorcerer's area where you can do other things as well. It's been a long time since I've been in there, but I love the Beast's library. Stay in there for a while as the room and the portrait and the magic rose change.
5. Ride the train and relax and enjoy the views of the park. Same for the monorail.
6. Wander the grounds of the Disneyland Hotel. We've done a little bit of that recently, and it's nice to see how everything's done. We went into Frontier (I think) Tower and happened to discover the model of Big Thunder Railroad in there. It's pretty awesome to look at.
7. Go through Tarzan's treehouse and see all of the special touches. Once you get up high, there's a nice view of the park from up there as well.
Hope that helps! Have a great visit.