This can probably go with the Twitter one, but I still think it's fun.
Like imagine this: Danny is working with some kid for a project and shows them a picture on his phone, something he saw at home that he thought might relate. Then, the kid pulls the classic jerk move and grabs Danny's phone and starts going through his photos before Danny can stop them. Eventually, the kid gets to a stupid group post-battle selfie of Danny, Sam, and Tucker. The kid pauses, looking up at Danny in horror because holy crap, that's Phantom, Danny Phantom, taking a picture with the goth girl and the techno geek. The kid suddenly realizes that the Danny sitting in front of him looks way too much like the Danny on the phone. Danny tries to do damage control. It does not work.
Alternatively: Danny takes a snapchat with the thermos, with some sort of dumb pun as the caption. He only means to send it to Sam and Tucker, maybe Jazz, when he accidentally puts it on his Story. He doesn't have a lot of people on snapchat, but he has a few who definitely are not on Team Phantom. Valerie suddenly gets a snap from Danny Fenton of the ghost boy with a goofy grin and the words: "just WRAPPED up a fight with the box ghost!"
Also: Another snapchat one, not really a revelation, but Danny is floating somewhere, not doing much, just messing around with his phone. Valerie sees Danny Phantom make a duck face at the camera. He glances up at her. "Alright, before you start shooting at me, can you think of a caption? I'm thinking '#nofilter.'" Eventually, it gets out that the ghost boy has a snapchat. People start asking him for his username.
