The internet has been speculating about a Euron Greyjoy/Cersei Lannister alliance since his brief return in Season 6, and Game of Thrones capitalized on it first thing in the Season 7 premiere. He promised Cersei the Iron Armada in exchange for her hand in marriage, but she declined. However, she desperately needs his naval power, so Euron thought he could sweeten the pot by delivering her an additional gift. So what is the gift Euron gives Cersei on Game of Thrones? We don’t know for sure, but there are a few compelling theories.
Many thought his fleet would be enough to satisfy Cersei, who, quite frankly, isn't really in a position to turn away allies at the moment. She destroyed the entire ruling class of King's Landing in a blaze of wildfire and then her last remaining child committed suicide. As she described to Jaime (in an elegant little narrative trick of exposition including a courtyard-sized walking map), the Lannisters are surrounded by enemies. And as Jaime reminded her, she is currently queen of maybe three kingdoms, at best, rather than the seven she claims in name only. But Euron's ships are not enough to woo Cersei. At least, not enough for her to want to marry him.
Some think he may be after a dragonhorn to give her. It is said that whoever possesses the dragonhorn can control Daenerys' dragons, which would be a critical ace in Cersei's pocket. Of course, she hasn't even seen Daenerys' dragons yet, and for all we know, she doesn't even believe that they're real.
Or, Euron could be after the Horn Of Winter to present to Cersei (yeah, magical horns are heavily thematized in this series). Legend has it that, thousands of years ago, a King-Beyond-The-Wall blew the Horn of Winter and awoke the first giants. Wildlings believe that blowing the horn again could topple The Wall. Based on the sheer visual spectacle of this option alone, I can't believe showrunners D.B. Weiss and David Benioff could resist setting off this particular plot device.
Otherwise, Euron could plan to kick it old school and just deliver Cersei the severed head of an enemy. Maybe Drogon, one of Daenerys' dragons. Maybe Tyrion, the brother she believes killed her son Joffrey and generally despises in every way possible. The options for heads Cersei would love on a spike are pretty endless, to be honest. Stay tuned to find out where Euron is headed and what he's looking for.