Go talk to Gabriella, who will tell you that the next target is Commander Maro's soon Gaius, a hit designed to put the commander's mind OFF of security. We will also plant a letter on Gaius making it look like HE wanted to kill the Emperor. Gaius Maro is set to go investigate cities in preparation for the Emperor's visit. We will get a bonus if we DON'T kill him in Dragon Bridge or on the ROAD, but in a city (as his body will be found faster). We also have the option of stealing his travel schedule.
Note: Getting the token results in also getting a MISCELLANEOUS quest that says " Receive a reading from Olava the Feeble ". While not really a random quest at all, I'll be listing it under the 'Dark Brotherhood Miscellaneous Quests' link just to put it somewhere. Check it out there.
This ends this questline, but as Gabrielle said there's been a disturbance with Cicero. You'll now start your next quest: "The Cure for Madness".
Head on over to Dragon Bridge where you'll see son and father part ways. Now, to get the OPTIONAL schedule head into the outpost. Chances are there's a guard in here (if there's not it's easy, or if he's sleeping it's easy). Even if he's sitting and watching the chair you can HIDE directly behind him and steal the schedule.
Now read the schedule and you'll see Gaius is a busy, BUSY guy! From here on in he's going to be making a trip around the world of Skyrim to inspect all of the cities. This trip will last the week, and once it's over he will do it all over again!
The easiest time to kill him is alone on the road, but we won't get our bonus this way. Instead, look at where he'll be SLEEPING in the next upcoming days... guard house, guard house, INN, etc... I'll tell you right now that no matter how badass you are you're not going to one-shot Gaius (well, I couldn't anyways and I THINK I'm badass... I imagine that if you have invested perks into the SNEAK skill, especially ASSASSIN'S BLADE, you'd be ok), so fighting him in a guard house isn't very smart. However, in the Bannered Mare Inn you can get into his room when he sleeps, shut the door, and essentially fight one on one! Crazy, you'd think someone would try to stop you, but no! And no bounty!
Note: He doesn't always go to his room to sleep and at exactly 12am on Lundi, he'll get up and start walking out of Whiterun. If you're he Yarl in Whiterun (by completing the earlier main missions here); you can kill him outside here and tell the guard to leave you alone, ensuring you don't have to worry about a bounty!
There's so many ways to kill Gaius that as long as you pull off the job you can't really go wrong. However, when you head back to the Dark Brotherhood Sanctuary, Gabrielle will not only give you 1000 gold for the dead, but if you killed him IN a city you will get OLAVA'S TOKEN as well.