Everyone keeps saying Bull Run, but I don't know why; the same reasons the Confederates failed to capitalized on their victory IOTL will be affecting the Federals here, in that they have an extremely Green force that cannot be expected to rapidly move to occupy strategic terrain before the defenders can rally. Manassas is over 100 miles from Richmond, after all.
Best way to have this happen is to have the
Virginia fail to dislodge her ram from the side of the
Cumberland, resulting in said ironclad going down with the Federals. Once the
Monitor arrives, they (local U.S. fleet) can begin clearing out the remaining Confederates, thus denying the Confederates the military stores in the area they had time to evacuate IOTL as well as securing the James River for use by McClellan's forces for the entirety of the campaign. Should the Army of the Potomac cut Petersburg, as Grant attempted to do in 1864, then the Confederates will have no choice but to abandon most of Virginia and her industries. Further to the West,
IOTL the Confederates almost didn't reinforce Vicksburg after the fall of New Orleans; presume in ATL they do not, as an overreaction to the loss of Norfolk and the perceived need to better defend Mobile. In such a case the city would fall, probably about the same time as the Confederates are forced to abandon Eastern Virginia. Thus, by the high summer of 1862, the Confederacy would be in the position of having lost its capital, most of its industry and would be bisected. In such a scenario I have no doubt they would begin peace talks thereafter, in order to end the war with the extremely lenient terms Lincoln still offered at this point.