Working outward from the meal: the simanim for the table, a honey dish and honey, and the tableware you plan to set. Then the things that make the days themselves — books and machzorim for the adults, crafts and activities for the children, and gifts for the hosts you will eat by.
Earlier than feels necessary. The stretch from the start of Elul is when everything sells, and anything that ships — books, honey gift sets, tableware — needs a delivery buffer. Crafts are the exception and are better done in the last week, when children are already thinking about the holiday.
Something for the table or the year ahead: artisan honey, a honey dish, a challah knife, or a book. All four travel well, need nothing from the host on the night, and are used rather than stored. A honey dish in particular gets brought out every year and remembered.
Give them something to make and something to do. The crafts here produce things that go on the table — a simanim card at each seat, a tzedakah box, a clay honey dish — so a child sees their own work in front of the family. The activities are for the younger ones who cannot yet sit through a craft.