I've used a number of fun apps as rewards for students when they complete their work. I have one student who is very interested in dinosaurs so I downloaded an app for him to learn more about the dinos. I also have a student who enjoys playing family feud, who doesn't. So when we have a break or after our class he will want to play that. Where's my water is another fun one that the kids enjoy.
I've used Facetime a couple times with kids at other schools and in the same school. I think that's an app that should be used more often.