1.When Tony removes his arc reactor and hands it to Steve, you can see a scar from the old version he had lodged in his chest.
2.Tony figures out his time traveling model after he draws "mild inspiration" from a photo of him and Peter, in which they're holding an inverted diploma.
3.You can spot a keg of Asgardian Ale in New Asgard.
4.Thor has Hulk and Iron Man PEZ dispensers in his room next to Stormbreaker.
5.When testing out the time machine, Hulk has to use a pencil to flip switches because his fingers are too big.
6.When we see Thor mention going for a "bit of lunch," it's a reference to the post-credit scene from The Avengers, where the group is seen eating shawarma.
15.Bucky tells Captain America he's going to miss him even though he's only supposed to be gone for seconds, because he knows him well enough to realize that Steve is going to live his life out, and this is the last time he'll see him young.