Game mat, books filled with random tables, spell cards, monster cards, other rpg books, boards games.

If it's in person, good pencils and/or pens

dice jails, dice towers, dice bags, dice jambalaya... wait, scratch that last one

supplemental books if you know they don't own them but might want them (for instance if they borrow a book you have a lot)

terrain minis for common settings, i.e. if you spend a lot of time in town get some tables, chairs, walls, doors, desks, bookshelves, etc.

similarly, if you encounter groups of a particular enemy frequently but don't have enough minis for all of them, i.e. goblins, orcs, regular town guards, wild animals, etc.