Renly Baratheon is dead, and while that news is still fresh Robb must choose the next course of action for himself as well as for the North. What does Robb decide?
Accept Margaery's offer to wed her and aim for the Iron Throne
Sitting the Iron Throne was never Robb's goal, and he doubted his father would have approved of him breaking his promise to marry one of Walder Frey's daughters. But however wary Robb may be of spending the rest of his life in the same city where his father was slain, the men, wealth and food that the Tyrells can offer give him the best chance of defeating the Lannisters.
And taking a woman as beautiful as Margaery as his wife and queen is nothing to be upset about either; the wedding and bedding are sure to be pleasant. It will take some doing to appease Lord Frey, and it's possible that not all of Robb's Northmen will be pleased with the idea of him going for the Iron Throne rather than ruling in Winterfell as the King in the North. But could there be any move he could make that would strengthen his chances of winning this war more than marrying Renly's widowed queen?
Return to Riverrun with his men (and Dacey)
Even if marrying Margaery would increase his chance significantly, Robb's only ever wanted to rule in Winterfell and that's what he's going to do. Plus he's betrothed to Walder Frey's daughter, and Robb was raised to be an honorable man who kept his word. He will stay the King in the North and fight for the Northern kingdomalone.
Dacey might find it amusing that he passed on a chance to take a woman as beautiful as Margaery into his bed, but Robb won't let things like that sway him. That doesn't mean he can't appreciate beautiful women though--or powerful ones either, for that matter.
Receive an interesting offer from Dorne
While Robb did not believe that Theon Greyjoy would betray them as his mother feared, he knew that Balon Greyjoy's last surviving son had been a ward/hostage in Winterfell for a reason. Rather than sending Theon back to the Iron Islands to attempt to negotiate an alliance, Robb instead sent him to Dorne. After all, wouldn’t the Martells still carry a grudge against the Lannisters for what had been done to Elia and her children?
Robb rejects Margaery's offer and returns to Riverrun, and while he plots his next move there he gets a raven from the south. The Martells had met with Theon, and though Prince Doran offers no formal response at this time, his brother Prince Oberyn (more known in some circles as the Red Viper) and daughter Princess Arianne would like to meet with Robb in person. Robb was primarily interested in seeing if Dorne would support him or at least not get in his way because of their mutual dislike for the Lannisters, but could the Red Viper and Prince Doran's heir be interested in creating an even stronger alliance between north and south?
Bend the knee to Stannis and his Red Woman
None of his men can understand how Robb would simply take off his crown and give up everything, but upon being convinced that Stannis is telling the truth about who fathered Cersei's children, he has to accept that Stannis is his king by rights. He doesn't know whether Stannis will truly force him to take the black and join the Night's Watch or if he will relent and allow him to remain as Lord of Winterfell, but Robb feels that he must choose honor over pride and renounce his claim of King in the North to swear Stannis fealty.
Before he officially takes off his crown though, the Red Woman Melisandre apparently has need for the seed of a king…
Attempt to negotiate an unlikely peace with the Lannisters
Considering that Robb initially called his banners because that prick Joffrey and Queen Cersei imprisoned his father and sisters, turning to them would seem like the last thing he would ever want to do. But while his father is dead, his sisters still live--and negotiating a peace with the Lannisters and exchanging them for the Kingslayer before Stannis inevitably attacks King's Landing might be the only way he can save them.
There's even a chance that the Lannisters, or more specifically Tywin, might see wisdom in granting the North their independence in exchange for them pulling out of the war and allowing him to focus on Stannis. That's more than he can expect from either Margaery who wanted him to take the Iron Throne or Stannis who will never suffer him claiming the title of King in the North. But could the Lannisters, much like Margaery, seek to go beyond a peace and seek a union through marriage?