One half term grandkids spent several days planning a picnic, writing the menu, writing a shopping list for required supplies, doing that shopping, baking a cake (battenburg! their choice!), making sandwiches, packing everything up including tablecloth and napkins, and then we drove them to the spot they chose. All very memorable and honestly if we had eaten it in the back garden that would have been fine, it was the planning and preparation they still remember. Eldest was 13 at the time.
Making an obstacle course in the garden, then organising the Olympics is also fun.