Well, before Lincoln actually assumed office, South Carolina had already asked for Ft Sumter to be handed over ( Jan. 31, 1861) and Buchanan had already dithered over and muddled reinforcements and resupply. When Lincoln assumed office, that situation was already in motion, there was nothing he could do, really, could not go back in time to plan what he would do if Ft Sumter were not fired upon.

A frequently-asked question here is, why didn't the Union just let the Southern states secede? Part of the reason why not is that there was more than simply announcing a separation. Ft. Sumter was just one thing within the Southern boundaries that was Federal property. Among other forts and properties, there was also the armory at Harper's Ferry, then in Virginia- at about the same time as Ft Sumter., the Confederacy would move to take that in April of 1861, capturing whatever arms had not first been burned by the Superintendant. And there was interstate infrastructure, like roads, railroads.... A critically important railroad connection from the mid-Atlantic states to the west ran through western Virginia, and the first land battle of the war would be over the control of it , in Philippi in June 1861.

And also very important were the western territories, which were going to become new states. These had already seen armed struggle for control, between pro-slavery and anti-slavery factions, and it continued. Confederate forces went into the Indian Territory ( now Oklahoma) and took Union forts, and the Native Nations there were soon squeezed between demands for military cooperation by the Confederacy and demands for resistance to it by the Union. Missouri devolved into its own civil war, Kansas continued to see battles between Bushwhackers and Jayhawks, as it had in the 1850's.

And there was also the Confederate states' readiness to start a fight. From the Missouri Compromise onwards, threats of secession and violence had been quite productive for them, and with these victories by the 1850's were encouraged to believe that every state had to accept slavery as legal. They had every reason to think that with more violence they could force the Union to negotiate something favorable to them once again. And, once real war had begun, they would continue to assume that.

So, other than Ft Sumter, there was plenty of the territory and property of the United States that the Union and Confederacy could fight over, there was great readiness of the Confederacy to start that fight, and, really, by the time Lincoln got to Washington, the fight had started. He had no reason to plan for what to do if there was a peaceful separation- it hadn't happened. And if the fight had not started at Ft Sumter, it would have soon been somewhere else.