Euron Greyjoy made quite the impression in the Season 7 premiere of Game of Thrones.
Mainly with the way he absolutely roasted Jaime.
He offered his armada to Cersei in return for her hand in marriage, which she declined. He left, vowing to return with a "priceless gift" to convince her to marry him.
We know from the Episode 3 description that Cersei will "return" a gift. It makes sense that it will be the one Euron gives her. Especially because it's highly unlikely she'll want to marry him, when she's finally obtained power in her own right.
But it will still be very interesting to see what Euron's gift will be – and what damage he might do to obtain it. Let's look at the options...
1. Tyrion Lannister
Cersei's desire for Tyrion's death is well-established. She's always hated him, and she also believes he's responsible for Joffrey's death and knows he killed Tywin. Tyrion has been a gift to a queen before (Dany, in Season 5), and his death would certainly be priceless to Cersei. But the chances of Euron getting near him within the next two episodes, when Tyrion is literally Dany's right-hand man, are pretty low.
2. Ellaria Sand and/or the Sand Snakes
Ellaria murdered Cersei's daughter Myrcella with the help of the Sand Snakes. Before this, they sent Cersei a "gift" of their own – Myrcella's necklace in the jaws of a viper, a clear threat on the girl's life. The Sand Snakes may have allied with Dany right now, but they're not in the inner circle, so Euron probably has a better chance of capturing one or all of them than he does of getting near Tyrion. It would also make the line about Cersei returning a gift in Episode 3 fitting – perhaps it's not Euron's gift she returns, but the Sand Snakes'. Cersei is all about vengeance these days, after all.
3. Olenna Tyrell
Cersei and Olenna hated each other even before Cersei blew up Olenna's family. With the Queen of Thorns now allied with Daenerys, her death would be something of value to Cersei and enable her to take over the Tyrell lands, and therefore the bulk of the food supply and riches left in the Seven Kingdoms. On the other hand, Olenna feels like kind of a random target from Euron's perspective.