She still knows magic, although she has lost all her magical potential after her transformation back to humanity. She'll need time to bring herself back up to what she was.
She invites Ice King and PB over many times out of the week. Its odd, at least when Ice King is there, but Simon has become a part of the family. The weird grandpa or uncle who sometimes needs help. Finn doesn't mind anymore. She's told him everything.
At night, Marceline tells Finn stories of what she remembers of the Old World. During the day, he takes her out to the ruins of a nearby cityscape, long since blasted into nothing but concrete skeletons of times gone by.
Together they assemble small things that would help them build their home up. An AMIGA 500, an old Zenith Television with a solar generator, a 1999 Toyota Corola that still works, because thats how 1999 Toyota Corolas do.
They get Floppy Disks from PB, who has programmed these ones to store at least ten gigabytes on them. They have ten. That's all they need.