Thor would open his without hesitation, and it'd be some Midgard candy

Bruce and Steve would both be a little more reluctant with theirs, but open them before too long. Bruce would probably get calming music and/or incense, and Cap some Trek nerd shit

Clint and Nat would throw their gifts in an incinerator, so he'd put sodium or something else that changes the color of fire. (they'd probably snark that they were expecting fireworks)

Tony would neither open his present or throw it away. he'd try to figure out what's inside it using his tech and ingenuity. so Loki would do everything he could to obscure the contents, and the box might well be empty, or only have ordinary rocks. or coal, if he wants to provoke Tony into ranting at the nerve of the former supervillain giving him coal