When Ray first meets Cassie she is wearing a necklace with a butterfly; possibly a nod to one of the most famous time travel short stories, 'A Sound of Thunder' by Ray Bradbury, where a time traveler changes the future (his present) by stepping on a butterfly in the distant past.
One of the most famous time travel short stories, 'A Sound of Thunder' by Ray Bradbury, where a time traveler changes the future (his present) by stepping on a butterfly in the distant past, is referenced during a discussion of the 'rules' of time travel, with Toby saying "You can't tread on any butterflies."
At the end of the film, as Cassie is trying to pull Ray into the portal, she says "We only have 14 hours to save the earth!" This is an obvious reference to the 1980 'Flash Gordon' movie, and is also featured in the Queen song of the same name.
Early on in the film, Pete and Toby press Ray for what his ideal profession would be, to which he replies "Time Lord" as a reference to the race of people from Doctor Who. Dean Lennox Kelly, who plays Pete (the time-travel skeptic), had previously portrayed William Shakespeare in the Doctor Who episode Doctor Who: The Shakespeare Code (2007).
The trivia items below may give away important plot points.
When Ray meets Cassie for the first time, right around the time they're about to leave, you can see a shadowy silhouette behind the glass door that's slightly cracked open. This is presumably when Ray is spying on the meeting later on in the film.